What does this PR do?

Adds all global tags to the top level trace tags hash map.

Working in conjunction with the apm-trace-intake team, intake will ensure that these tags are placed onto all spans in the trace and then are made available for our usage metric.

Motivation

Part of the usage metrics tagging which will allow us to include more tags (including custom tags) on the datadog.serverless.traced_invocations metric.

Additional Notes

Note that this change will duplicate the location of the global tags in the trace payload sent to intake. Currently, these payloads look like

TracePayload {
  ...
  Chunk {
    ...
    Span {
      ...
      Meta {
        ...
        "key1": "value1",
        "key2": "value2",
        "key3": "value3"
      }
    }
  }
  Tags {}
}

After this change they will look like

TracePayload {
  ...
  Chunk {
    ...
    Span {
      ...
      Meta {
        ...
        "key1": "value1",
        "key2": "value2",
        "key3": "value3"
      }
    }
  }
  Tags {
    "_dd.tags.function": "key1:value1,key2:value2,key3:value3"
  }
}

Once the changes are in place in intake, we can remove the tags from the spans themselves. The intake pipeline will manage unpacking and copying these tags for us, at which point trace payloads will look like

TracePayload {
  ...
  Chunk {
    ...
    Span {
      ...
      Meta {
        ...
      }
    }
  }
  Tags {
    "_dd.tags.function": "key1:value1,key2:value2,key3:value3"
  }
}