Log traceID and meta RenderStats

[shanson7]

Fixes #1973

[Dieterbe]

curious if @joe-elliott has any suggestions here.
Joe, if we want to emit log messages and somehow correlate messages corresponding to the same request together, what would you recommend? I know in jaeger land, you'd attach the log messages to the trace (i believe they get sent in the span data) but that makes us depend on jaeger, which is a heavy/expensive dependency we rather not enforce.
Shall we just log the trace ID in the log message and write it out to a file/stdout? AFAIK loki doesn't have a way to join messages by traceID (but it does have a way to filter by trace ID, and we could join in a log analysis script). Does tempo try to solve this at all?

[joe-elliott]

Tempo also stores and returns "logs" or "events" like Jaeger. So you can continue attaching your logs to the spans and they will be retrieved when you request the trace.

[Dieterbe]

this LGTM. just one outstanding comment about potentially moving the log call into renderMetrics()