Fixes traces per page.

[chris-ramon]

Details

Issue: When visiting /traces it's not limit the number of traces per page(Current default: 10 traces).

• [x] Improvements on InfluxDBStore.Traces(...):
  • Previous query included GROUP BY * LIMIT 10 together, but turns out those keywords does not work well together.
  • We've being using GROUP BY * to obtain tags, in our case span's ids(trace, span, parent).
  • We can have a clearer query by removing GROUP BY * and more important having LIMIT back working.
  • New strategy: obtains span's ids from the query result by checking if it is an actual "trace_id", "span_id" or "parent_id"; here the relevant code.
• [x] Cleans up code related to new strategy for obtaining span's id from queries result.

[slimsag]

LGTM