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?