autoexport: Add support for metrics

[punya]

Fixes #4131.

Reorganizes autoexport into

• generic registry and env lookup independent of signal type
• metric- and trace-specific wrappers around the generic part, to preserve a simple API
• pre-defined metric and trace factories

Also reorganizes tests to line up with these layers.

[codecov]

Codecov Report

Merging #4229 (503e4ff) into main (72a4bcb) will decrease coverage by 0.1%.
Report is 1 commits behind head on main.
The diff coverage is 86.9%.

@@           Coverage Diff           @@
##            main   #4229     +/-   ##
=======================================
- Coverage   80.8%   80.8%   -0.1%     
=======================================
  Files        148     150      +2     
  Lines      10238   10245      +7     
=======================================
+ Hits        8281    8284      +3     
- Misses      1819    1823      +4     
  Partials     138     138             

Files	Coverage Δ
exporters/autoexport/registry.go	100.0% <100.0%> (ø)
exporters/autoexport/signal.go	100.0% <100.0%> (ø)
exporters/autoexport/noop.go	66.6% <75.0%> (+23.8%)	⬆️
exporters/autoexport/spans.go	90.9% <90.9%> (ø)
exporters/autoexport/metrics.go	73.3% <73.3%> (ø)

[pellared]

I just made a very quick look. I would need to look deeper in the following days. Looks nice so far👍

[pellared]

Looks very good 👍 I have found only one corner case issue.

[punya]

@pellared @MrAlias this is ready for another round of review.

[dashpole]

From https://github.com/open-telemetry/opentelemetry-specification/blob/563958cb2bd8529990f19fdce7a5f3643bf63091/specification/configuration/sdk-environment-variables.md?plain=1#L222:

The SDK MAY accept a comma-separated list to enable setting multiple exporters.

This would be useful, but doesn't really fit with how the package is currently designed.

[MrAlias]

Should this be TraceOption?

I know we have the SpanExporter, but I wonder if this needs to match that or the corollary to metric: trace.

[pellared]

We can address it in a separate PR if needed.

Personally, I was thinking about renaming to SpanExporterOption.

Let's track it in a separate issue.

[pellared]

#4474

[pellared]

From https://github.com/open-telemetry/opentelemetry-specification/blob/563958cb2bd8529990f19fdce7a5f3643bf63091/specification/configuration/sdk-environment-variables.md?plain=1#L222:

The SDK MAY accept a comma-separated list to enable setting multiple exporters.

This would be useful, but doesn't really fit with how the package is currently designed.

I created #4471