feat: add tags to config

[apiarian-datadog]

What does this PR do?

Adding function tags for traces processed by the the datadog-trace-agent.

Trace metrics don't have these tags yet (are they computed in the tracer for serverless-components?)

Motivation

This will allow the backend to index traces by these trace tags, enabling trace search queries like tag_name:value rather than @tag_name:value (the latter would search for span meta attributes instead).

Additional Notes

This has previously been implemented in the datadog-lambda-extension bottlecap agent and the serverless-init build of the datadog-agent.

Describe how to test/QA your changes

Added unit tests. Also deployed to a test app to confirm that these function trace tags are being added and result in searchable traces.

[duncanpharvey]

Following up on @lucaspimentel's comment around comma and space separation - I think we should handle both cases. See below for an example implementation in the Node Tracer:

Convert space separation to comma separation: https://github.com/DataDog/dd-trace-js/blob/99e7bb4b0977de2daf85f0ddf9af5b3d420bb0db/packages/dd-trace/src/config.js#L827

Then parse comma separated tags: https://github.com/DataDog/dd-trace-js/blob/99e7bb4b0977de2daf85f0ddf9af5b3d420bb0db/packages/dd-trace/src/tagger.js#L26

Otherwise the comma separated tags work as expected DD_TAGS = "tagkey1:tagval1,tagkey2:tagval2" except in the case of filtering in the live trace view.

[apiarian-datadog]

thanks @duncanpharvey! the lack of tag query in the live view is expected since the tags aren't applied to the traces a that point in our backend systems.

i've added space-separated tag support as well.