processor/otel: extend support for OpenTelemetry semantic conventions

[axw]

Motivation/summary

Extend support for OpenTelemetry semantic conventions to cover what we support for traces in the OpenTelemetry Collector exporter, minus support for "exception" span events. These will be covered in a followup.

A couple of functional changes for the existing Jaeger support:

• If the span type is not inferred, it will now be "app" rather than "custom". This aligns with the OpenTelemetry Collector exporter, and works better with the "time spent by span type" breakdown charts.
• We no longer assume an outcome/result of "success" if the span didn't fail. We now default to "unknown", and only set "succcess" or "failure" if one of those outcomes is known, like we do in the OpenTelemetry Collector exporter and our own agents.

Checklist

• Update CHANGELOG.asciidoc
  - [ ] Documentation has been updated [OTLP] Document support for OTLP/gRPC on the standard port #4678

How to test these changes

1. Start apm-server
2. Run an OpenTelemetry-instrumented application, configured to export OTLP to APM Server's standard host:port.
3. Repeat step 2, exporting to an OpenTelemetry Collector with the elastic exporter.
4. Compare the data received directly by APM Server to the data sent by the exporter.

Related issues

#4503

[apmmachine]

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Build stats

• Build Cause: Pull request #4711 updated

• Start Time: 2021-02-11T01:08:42.461+0000

• Duration: 40 min 50 sec

• Commit: a9d0473

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Passed	4721
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Total	4845

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[jalvz]

Didn't run it, but LGTM. That's going to be a lot of fields for testing...

[simitt]

Tested with BC 2 - works as expected.

Tested with the otel elastic exporter, otel otel/elastic exporter and directly exporting from a go app instrumented with the otel exporter. Stored information on transactions was the same.