Improvements on `D3 Timeline` data.

[chris-ramon]

Details

• [x] Adds empty checking to event.Start() & event.End() values, so no empty times are added and later returned to the frontend where it's being use to draw d3 timeline chart, which would crash when trying to use an empty value from times.

Notes:

• I've discovered this while working on #99:
• Based on examples/cmd/webapp/main.go:
  • MemoryStore - won't save _schema:HTTPClient Event for root spans.
  • When building d3 timeline data for a given root span - everything is fine, all other events different than (_schema:HTTPClient) are present & contains values.
• Based on examples/cmd/webapp-influxdb/main.go:
  • InfluxDBStore - won't save _schema:HTTPClient Event for root spans.
  • When building d3 timeline data for a given root span, event: _schema:HTTPClient & it's associated times are present & it's values are empty, not because we did saved but because InfluxDB will include in the query result all the fields from the measurement - and the field _schema:HTTPClient was created for other spans.
  • We can think about it as: First we write to DB a new root span which does not contain _schema:HTTPClient as field so obviously it is not created on spans measurement - later a child span is saved to DB which does have _schema:HTTPClient as field, so this new field is created and for the existing spans the value of this new field is set to empty - said that, when doing queries like SELECT * FROM SPANS WHERE trace_id=ABC - will include all fields for measurement spans all those used when inserting trace_id=ABC and the ones that weren't, those last one with empty values.

[slimsag]

Who emits an empty TimespanEvent in the first place? It could be confusing if someone emits a span that does not have a timespan, and then it does not show up in the frontend UI.

Instead, we should probably catch this at the moment of intent. I.e. if someone emits a TimespanEvent with empty times we log an error / ignore it there (they could insert a panic to track down the bug, but panic is not good in production).