Add-AzureRmMetricAlertRule failing for function app metrics

### Cmdlet(s)
Add-AzureRmMetricAlertRule

### PowerShell Version
5.1.16299.19

### Module Version
4.0.0

### OS Version
10.0.16299.19

### Description
When I try to add the "FunctionExecutionCount" or "Http5xx" metrics to a function app using the Add-AzureRmMetricAlertRule, it fails citing the error "Add-AzureRmMetricAlertRule : Exception type: ErrorResponseException, Message: The metric with namespace '' and name 'FunctionExecutionCount' is not supported for this resource id". The same behavior is observed for function apps with an App Service Plan and a Consumption Plan.

### Debug Output
DEBUG: 4:46:55 PM - AddAzureRmMetricAlertRuleCommand begin processing with ParameterSet '__AllParameterSets'.
DEBUG: 4:46:55 PM - using account id 'vinaydami@outlook.com'...
WARNING: [Add-AzureRmMetricAlertRule] Parameter name change: The parameter plural names for the parameters will be deprecated in a future breaking change
release in favor of the singular versions of the same names.
DEBUG: [Common.Authentication]: Authenticating using Account: 'vinaydami@outlook.com', environment: 'AzureCloud', tenant: '5ed1d584-3cdd-4f8b-87b5-515a23
e7e65e'
DEBUG: Microsoft.IdentityModel.Clients.ActiveDirectory Information: 2 :
DEBUG: 11/14/2017 11:16:55: 16a2e83f-93d5-4471-a9b7-f34e810934ef - AcquireTokenHandlerBase: === Token Acquisition started:
Authority: https://login.microsoftonline.com/5ed1d584-3cdd-4f8b-87b5-515a23e7e65e/
Resource: https://management.core.windows.net/
ClientId: 1950a258-227b-4e31-a9cf-717495945fc2
CacheType: Microsoft.Azure.Commands.Common.Authentication.AuthenticationStoreTokenCache (6 items)
Authentication Target: User

DEBUG: Microsoft.IdentityModel.Clients.ActiveDirectory Verbose: 1 :
DEBUG: 11/14/2017 11:16:55: 16a2e83f-93d5-4471-a9b7-f34e810934ef - TokenCache: Looking up cache for a token...

DEBUG: Microsoft.IdentityModel.Clients.ActiveDirectory Information: 2 :
DEBUG: 11/14/2017 11:16:55: 16a2e83f-93d5-4471-a9b7-f34e810934ef - TokenCache: An item matching the requested resource was found in the cache

DEBUG: Microsoft.IdentityModel.Clients.ActiveDirectory Verbose: 1 :
DEBUG: 11/14/2017 11:16:55: 16a2e83f-93d5-4471-a9b7-f34e810934ef - TokenCache: 29.7242895766667 minutes left until token in cache expires

DEBUG: Microsoft.IdentityModel.Clients.ActiveDirectory Information: 2 :
DEBUG: 11/14/2017 11:16:55: 16a2e83f-93d5-4471-a9b7-f34e810934ef - TokenCache: A matching item (access token or refresh token or both) was found in the c
ache

DEBUG: Microsoft.IdentityModel.Clients.ActiveDirectory Information: 2 :
DEBUG: 11/14/2017 11:16:55: 16a2e83f-93d5-4471-a9b7-f34e810934ef - AcquireTokenHandlerBase: === Token Acquisition finished successfully. An access token
was retuned:
Access Token Hash: vqZ9QV1qyjNrjiISc/Ym8W7r3RSLJpKelruP+3vwbTg=
Refresh Token Hash: VDR4u6C0ZYqv1GBz/FdeF9w0pTZ35mMuhTOIrf9s/Sg=
Expiration Time: 11/14/2017 11:46:38 +00:00
User Hash: 4uMjdux6PlbHohqQSPfTCHKRPTSuusBeoflGpyYQ1ps=

DEBUG: Microsoft.IdentityModel.Clients.ActiveDirectory Information: 2 :
DEBUG: 11/14/2017 11:16:55: - TokenCache: Serializing token cache with 6 items.

DEBUG: Microsoft.IdentityModel.Clients.ActiveDirectory Information: 2 :
DEBUG: 11/14/2017 11:16:55: 6fbd5187-9561-4284-9b57-1c3b5512956d - AcquireTokenHandlerBase: === Token Acquisition started:
Authority: https://login.microsoftonline.com/5ed1d584-3cdd-4f8b-87b5-515a23e7e65e/
Resource: https://management.core.windows.net/
ClientId: 1950a258-227b-4e31-a9cf-717495945fc2
CacheType: Microsoft.Azure.Commands.Common.Authentication.AuthenticationStoreTokenCache (6 items)
Authentication Target: User

DEBUG: Microsoft.IdentityModel.Clients.ActiveDirectory Verbose: 1 :
DEBUG: 11/14/2017 11:16:55: 6fbd5187-9561-4284-9b57-1c3b5512956d - TokenCache: Looking up cache for a token...

DEBUG: Microsoft.IdentityModel.Clients.ActiveDirectory Information: 2 :
DEBUG: 11/14/2017 11:16:55: 6fbd5187-9561-4284-9b57-1c3b5512956d - TokenCache: An item matching the requested resource was found in the cache

DEBUG: Microsoft.IdentityModel.Clients.ActiveDirectory Verbose: 1 :
DEBUG: 11/14/2017 11:16:55: 6fbd5187-9561-4284-9b57-1c3b5512956d - TokenCache: 29.7241557933333 minutes left until token in cache expires

DEBUG: Microsoft.IdentityModel.Clients.ActiveDirectory Information: 2 :
DEBUG: 11/14/2017 11:16:55: 6fbd5187-9561-4284-9b57-1c3b5512956d - TokenCache: A matching item (access token or refresh token or both) was found in the c
ache

DEBUG: Microsoft.IdentityModel.Clients.ActiveDirectory Information: 2 :
DEBUG: 11/14/2017 11:16:55: 6fbd5187-9561-4284-9b57-1c3b5512956d - AcquireTokenHandlerBase: === Token Acquisition finished successfully. An access token
was retuned:
Access Token Hash: vqZ9QV1qyjNrjiISc/Ym8W7r3RSLJpKelruP+3vwbTg=
Refresh Token Hash: VDR4u6C0ZYqv1GBz/FdeF9w0pTZ35mMuhTOIrf9s/Sg=
Expiration Time: 11/14/2017 11:46:38 +00:00
User Hash: 4uMjdux6PlbHohqQSPfTCHKRPTSuusBeoflGpyYQ1ps=

DEBUG: Microsoft.IdentityModel.Clients.ActiveDirectory Information: 2 :
DEBUG: 11/14/2017 11:16:55: - TokenCache: Serializing token cache with 6 items.

DEBUG: ============================ HTTP REQUEST ============================

HTTP Method:
PUT

Absolute Uri:
https://management.azure.com/subscriptions/8d6baaa6-0ec4-49fd-acfa-55c3194e32dd/resourcegroups/functions/providers/microsoft.insights/alertrules/functest
?api-version=2016-03-01

Headers:
x-ms-client-request-id : f11fbc53-a7a8-4567-ba09-cc123809fe9e
accept-language : en-US

Body:
{
"properties": {
"name": "functest",
"description": "This metric alert rule was created from Powershell version: 4.0.0",
"isEnabled": true,
"condition": {
"odata.type": "Microsoft.Azure.Management.Insights.Models.ThresholdRuleCondition",
"operator": "GreaterThan",
"threshold": 100.0,
"windowSize": "PT5M",
"timeAggregation": "Total",
"dataSource": {
"odata.type": "Microsoft.Azure.Management.Insights.Models.RuleMetricDataSource",
"metricName": "FunctionExecutionCount",
"resourceUri": "/subscriptions/8d6baaa6-0ec4-49fd-acfa-55c3194e32dd/resourceGroups/functions/providers/Microsoft.Web/sites/funcappstdplan"
}
}
},
"location": "westus",
"tags": {
"$type": "Microsoft.WindowsAzure.Management.Common.Storage.CasePreservedDictionary,Microsoft.WindowsAzure.Management.Common.Storage",
"hidden-link:/subscriptions/8d6baaa6-0ec4-49fd-acfa-55c3194e32dd/resourceGroups/functions/providers/Microsoft.Web/sites/funcappstdplan": "Resource"
}
}

DEBUG: ============================ HTTP RESPONSE ============================

Status Code:
BadRequest

Headers:
Pragma : no-cache
Strict-Transport-Security : max-age=31536000; includeSubDomains
x-ms-request-id : 00000000-0000-0000-0000-000000000000
x-ms-ratelimit-remaining-subscription-writes: 1195
x-ms-correlation-request-id : 0a9e5112-d5f7-43b6-b346-8d82ca7c83ba
x-ms-routing-request-id : SOUTHINDIA:20171114T111700Z:0a9e5112-d5f7-43b6-b346-8d82ca7c83ba
Cache-Control : no-cache
Date : Tue, 14 Nov 2017 11:16:59 GMT
Server : Microsoft-IIS/8.5

Body:
{
"code": "UnsupportedMetric",
"message": "The metric with namespace '' and name 'FunctionExecutionCount' is not supported for this resource id '/subscriptions/8d6baaa6-0ec4-49fd-acf
a-55c3194e32dd/resourceGroups/functions/providers/Microsoft.Web/sites/funcappstdplan'."
}

Add-AzureRmMetricAlertRule : Exception type: ErrorResponseException, Message: The metric with namespace '' and name 'FunctionExecutionCount' is not
supported for this resource id
'/subscriptions/8d6baaa6-0ec4-49fd-acfa-55c3194e32dd/resourceGroups/functions/providers/Microsoft.Web/sites/funcappstdplan'., Code: UnsupportedMetric,
Status code:BadRequest, Reason phrase: Bad Request
At line:1 char:1
+ Add-AzureRmMetricAlertRule -ResourceGroup functions -TargetResourceId ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : CloseError: (:) [Add-AzureRmMetricAlertRule], PSInvalidOperationException
+ FullyQualifiedErrorId : Microsoft.Azure.Commands.Insights.Alerts.AddAzureRmMetricAlertRuleCommand

DEBUG: AzureQoSEvent: CommandName - Add-AzureRmMetricAlertRule; IsSuccess - False; Duration - 00:00:05.1417806; Exception - System.Management.Automation.
PSInvalidOperationException: Exception type: ErrorResponseException, Message: The metric with namespace '' and name 'FunctionExecutionCount' is not suppo
rted for this resource id '/subscriptions/8d6baaa6-0ec4-49fd-acfa-55c3194e32dd/resourceGroups/functions/providers/Microsoft.Web/sites/funcappstdplan'., C
ode: UnsupportedMetric, Status code:BadRequest, Reason phrase: Bad Request ---> Microsoft.Azure.Management.Monitor.Management.Models.ErrorResponseExcepti
on: Operation returned an invalid status code 'BadRequest'
at Microsoft.Azure.Management.Monitor.Management.AlertRulesOperations.d__5.MoveNext()
--- End of inner exception stack trace ---
at Microsoft.Azure.Commands.Insights.MonitorCmdletBase.ExecuteCmdlet()
at Microsoft.WindowsAzure.Commands.Utilities.Common.AzurePSCmdlet.ProcessRecord();
DEBUG: Finish sending metric.
DEBUG: 4:47:00 PM - AddAzureRmMetricAlertRuleCommand end processing.
DEBUG: 4:47:00 PM - AddAzureRmMetricAlertRuleCommand end processing.

### Script/Steps for Reproduction
1) Create a function app with a Standard App service plan or a consumption plan
2) Execute the following powershell command:
Add-AzureRmMetricAlertRule -ResourceGroup functions -TargetResourceId "/subscriptions/8d6baaa6-0ec4-49fd-acfa-55c3194e32dd/resourceGroups/functions/providers/Microsoft.Web/sites/funcappstdplan" -MetricName "FunctionExecutionCount" -Name "functest" -WindowSize 00:05:00 -TimeAggregationOperator Total -Operator GreaterThan -Threshold "100" -Location "westus"