block_blob_service.set_blob_metadata replaces existing properties - change default behaviour

### Which service(blob, file, queue) does this issue concern?

blob

### Which version of the SDK was used? Please provide the output of `pip freeze`.

azure==4.0.0
azure-applicationinsights==0.1.0
azure-batch==4.1.3
azure-cognitiveservices-vision-customvision==1.0.0
azure-common==1.1.20
azure-cosmosdb-nspkg==2.0.2
azure-cosmosdb-table==1.0.5
azure-datalake-store==0.0.45
azure-eventgrid==1.2.0
azure-functions==1.0.0b4
azure-functions-worker==1.0.0b6
azure-graphrbac==0.40.0
azure-keyvault==1.1.0
azure-loganalytics==0.1.0
azure-mgmt==4.0.0
azure-mgmt-advisor==1.0.1
azure-mgmt-applicationinsights==0.1.1
azure-mgmt-authorization==0.50.0
azure-mgmt-batch==5.0.1
azure-mgmt-batchai==2.0.0
azure-mgmt-billing==0.2.0
azure-mgmt-cdn==3.1.0
azure-mgmt-cognitiveservices==3.0.0
azure-mgmt-commerce==1.0.1
azure-mgmt-compute==4.6.2
azure-mgmt-consumption==2.0.0
azure-mgmt-containerinstance==1.4.1
azure-mgmt-containerregistry==2.8.0
azure-mgmt-containerservice==4.4.0
azure-mgmt-cosmosdb==0.4.1
azure-mgmt-datafactory==0.6.0
azure-mgmt-datalake-analytics==0.6.0
azure-mgmt-datalake-nspkg==3.0.1
azure-mgmt-datalake-store==0.5.0
azure-mgmt-datamigration==1.0.0
azure-mgmt-devspaces==0.1.0
azure-mgmt-devtestlabs==2.2.0
azure-mgmt-dns==2.1.0
azure-mgmt-eventgrid==1.0.0
azure-mgmt-eventhub==2.6.0
azure-mgmt-hanaonazure==0.1.1
azure-mgmt-iotcentral==0.1.0
azure-mgmt-iothub==0.5.0
azure-mgmt-iothubprovisioningservices==0.2.0
azure-mgmt-keyvault==1.1.0
azure-mgmt-loganalytics==0.2.0
azure-mgmt-logic==3.0.0
azure-mgmt-machinelearningcompute==0.4.1
azure-mgmt-managementgroups==0.1.0
azure-mgmt-managementpartner==0.1.0
azure-mgmt-maps==0.1.0
azure-mgmt-marketplaceordering==0.1.0
azure-mgmt-media==1.0.0
azure-mgmt-monitor==0.5.2
azure-mgmt-msi==0.2.0
azure-mgmt-network==2.7.0
azure-mgmt-notificationhubs==2.1.0
azure-mgmt-nspkg==3.0.2
azure-mgmt-policyinsights==0.1.0
azure-mgmt-powerbiembedded==2.0.0
azure-mgmt-rdbms==1.8.0
azure-mgmt-recoveryservices==0.3.0
azure-mgmt-recoveryservicesbackup==0.3.0
azure-mgmt-redis==5.0.0
azure-mgmt-relay==0.1.0
azure-mgmt-reservations==0.2.1
azure-mgmt-resource==2.1.0
azure-mgmt-scheduler==2.0.0
azure-mgmt-search==2.0.0
azure-mgmt-servicebus==0.5.3
azure-mgmt-servicefabric==0.2.0
azure-mgmt-signalr==0.1.1
azure-mgmt-sql==0.9.1
azure-mgmt-storage==2.0.0
azure-mgmt-subscription==0.2.0
azure-mgmt-trafficmanager==0.50.0
azure-mgmt-web==0.35.0
azure-nspkg==3.0.2
azure-servicebus==0.21.1
azure-servicefabric==6.3.0.0
azure-servicemanagement-legacy==0.20.6
azure-storage-blob==1.5.0
azure-storage-common==1.4.2
azure-storage-file==1.4.0
azure-storage-queue==1.4.0
msrestazure==0.6.0

### What problem was encountered?

#390 refers. The default behaviour is for set_blob_metadata to *replace* existing metadata. While a workaround is to get then set, this is still extremely dangerous behaviour.

I don't know what the wider storage behaviour is and hence whether the two are consistent.

### Have you found a mitigation/solution?

This is a feature request, but see above:

I would like to see a keyword argument to merge/update/upsert by default, or to warn at the very least. Or to snapshot?

Thoughts?

Start with the block_blob_service.set_blob_metadata entry point and review issue #390 plus the Azure Blob Storage metadata semantics. Determine whether merge, update, warning, or snapshot behavior is consistent with the wider storage API, then define and test the chosen default behavior and its compatibility impact.

Written by the indexing model from the issue text.