UserAgent: Fix feature tracking

[ealsur]

Description

Due to the underlying inheritance from Cosmos.UserAgentContainer and Documents.UserAgentContainer, the base user agent string only gets re-calculated if the Suffix (ApplicationName) setter is used on the underlying Documents.UserAgentContainer.

Currently we are tracking a set of Features through the user Agent, but since these Features are set during Client creation, cannot be part of the BaseUserAgent. So we set them after, like the Suffix, when building the ConnectionPolicy:

https://github.com/Azure/azure-cosmos-dotnet-v3/blob/master/Microsoft.Azure.Cosmos/src/CosmosClientOptions.cs#L748

The problem is that if the user is not setting ApplicationName / Suffix, and using features like Bulk or the new HttpClientFactory, those features do not get added to the UserAgent because the Suffix is not modified.

Calling the Suffix setter with a string.Empty forces the base user agent to be recalculated. This does not affect the scenario where the user is indeed setting the ApplicationName because that happens after.

Since this only happens on client building, there is no performance impact.

Ideal solution would be to stop inheriting from Documents.UserAgentContainer but that would require a rewiring through the complete stack including files in the Direct package.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)