Suggestion to add telemetry when sending 400 response code for invalid activity

[ninilang]

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Is your feature request related to a problem? Please describe.
Currently, the SDK returns a 400 response code when the incoming activity is null or the activity type is empty. This is the piece of code being referred to. Since no trace is logged here, it's difficult to know if this piece of code actually caused the 400 result code.

Describe the solution you'd like
A trace indicating that we returned a 400 response here will make it easier while look at telemetry.

Describe alternatives you've considered
N/A

Additional context

[mrivera-ms]

@ninilang, can you indicate what information you need added in the message?