feat(sdk-node)!: do not instantiate metrics SDK on empty metricReaders

[pichlermarc]

Which problem is this PR solving?

There's currently no programmatic way to tell the SDK not to instantiate a Metrics SDK. For traces this can be done via spanProcessors: []. We should follow the same behavior for metrics, where

• metricReaders: [] -> instantiates no metrics SDK, no matter what's set in OTEL_METRICS_EXPORTER

This change is breaking only for users that use metricReaders: [] AND OTEL_METRICS_EXPORTER=... at the same time.

Ref #6092

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.39%. Comparing base (228cb92) to head (945a3af).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6272      +/-   ##
==========================================
- Coverage   95.40%   95.39%   -0.01%     
==========================================
  Files         316      316              
  Lines        9568     9558      -10     
  Branches     2214     2210       -4     
==========================================
- Hits         9128     9118      -10     
  Misses        440      440              

Files with missing lines	Coverage Δ
...imental/packages/opentelemetry-sdk-node/src/sdk.ts	96.35% <100.00%> (-0.19%)	⬇️

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[cjihrig]

You could use optional chaining here and drop the first line of this check.

[pichlermarc]

yeah, I found the optional chaining to be a bit more difficult to read compared so I went with the other thing.

(this._meterProviderConfig?.readers?.length ?? 0) > 0

[cjihrig]

It looks like this didn't actually get changed.

[JamieDanielson]

Small update in wording for the changelog (typo)