Grpc channel shutdown can cause license_key is required error message.

[vuqtran88]

Description
When something causes the grpc channel disconnect, the agent tries to shutdown and reconnect. It also resets the metadata object which has the license key header in it. This line of code causes the issue:

newrelic-dotnet-agent/src/Agent/NewRelic/Agent/Core/DataTransport/DataStreamingService.cs

Line 626 in c75eeb0

MetadataHeaders = null;

.

Even though when the agent reconnects, the MetadataHeaders will get set again. The problem is, during the connection shutdown, the agent had a bunch of streaming workers that were still streaming data with the cleared MetadataHeaders.

A potential fix is here https://github.com/newrelic/newrelic-dotnet-agent/tree/fix-license-key-header-issue.

Expected Behavior
Streaming worker should not use null value MetadataHeaders to send spans when the channel connection is restarting.