Show metric for Trigger Work Item Count in Dedicated Functions Applications

As discussed with @mhoeger .

**Issue**
When customers are seeing performance issues or failed to trigger issues on Dedicated Function apps we rely on the external trigger work item count to validate if they are right.

**Possible Solution**
Look at emitting some type of metric to show pending work for each of the trigger types.

E.G for CosmosDB there's already some work in the Change Feed Processor that we can use.
[https://medium.com/microsoftazure/azure-cosmos-db-functions-cookbook-monitoring-trigger-pending-work-800b24589235
](url)

Start by reviewing the existing external trigger work item count and the Change Feed Processor work mentioned in the issue. Define which trigger types and pending-work values should be exposed, then verify that Dedicated Function apps emit a usable metric for the supported triggers.

Written by the indexing model from the issue text.