Consolidate OTLP variables to use TRACES / METRICS to describe talking…

[anuraaga]

… to TraceService / MetricsService

Figured we need a PR to try to knock these out :)

Fixes #1352
Fixes #1353

Changes

Environment variables for configuring OTLP exporters are changed from SPAN / METRIC to TRACE / METRICS. This is to correspond to the services being connected to, TraceService and MetricsService. It seems natural, and leaves out ambiguity by just matching up with the service names. Hopefully these variables won't be used all that much anyways in favor of the polyglot variables.

[carlosalberto]

I'm a little bit inclined to go with SPAN instead of TRACE (as, I think, all SIGs already use SpanExporter, just as the Specification itself does, as opposed to TraceExporter). Not a strong opinion, though.

[tedsuo]

We discussed in the Maintainers meeting, and chose TRACE.

[reyang]

LGTM.

[carlosalberto]

@open-telemetry/specs-metrics-approvers Please somebody give us a blessing here, as Metrics is also updated ;)

[tigrannajaryan]

OTLP uses plural traces, metrics and logs for consistency:
https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/protocol/otlp.md

[anuraaga]

OTLP uses plural traces, metrics and logs for consistency:

@tigrannajaryan Really? I only see trace in the package, file, and service name so think it is closer

https://github.com/open-telemetry/opentelemetry-proto/tree/master/opentelemetry/proto

[tigrannajaryan]

OTLP uses plural traces, metrics and logs for consistency:

@tigrannajaryan Really? I only see trace in the package, file, and service name so think it is closer

https://github.com/open-telemetry/opentelemetry-proto/tree/master/opentelemetry/proto

We cannot change those name since it breaks compatibility and they are already declared stable.

However, as far as I remember this was discussed and a decision was made that we should use plural for all signals going forward where we can. Collector codebase uses plural traces everywhere. OTLP/HTTP uses /v1/traces as the default URL.

[carlosalberto]

After a second look, agreed that we should go for TRACES, as keeping all signals plural is easier to remember IMHO (besides the compatibility reasons @tigrannajaryan gave).

[bogdandrutu]

+1 for @tigrannajaryan comments

[anuraaga]

Updated