Exemplars should treat trace_id and span_id as fields instead of tags.

[rmelick-muon]

We recently deployed auto instrumentation of the python requests library to one of our services, and noticed that this caused our cardinality within Influx to explode. We traced the issue down to the metric http.client.duration_exemplar, and the two tags trace_id and span_id. Since the trace id and span id are unique values, I think that treating them as tags is violating the recommended schema design, and they should be set as fields instead.