Update OpenCensus shim metrics docs to use registerMetricProducer

[thswlsqls]

Fixes #8811

Description

• The "Metrics" examples in opencensus-shim/README.md still call OpenCensusMetrics.attachTo(reader); that class was removed in Stabilize MetricProducer, allow custom MetricReaders #5835 (2023-09), so the snippets do not compile.
• The first example also called SdkMeterProviderBuilder.buildAndRegisterGlobal(), which was removed in Remove deprecated metrics APIs #3990 and only exists on OpenTelemetrySdkBuilder.
• Both examples now use registerMetricReader(reader).registerMetricProducer(OpenCensusMetricProducer.create()).build(), as in OpenCensusMetricsTest and the class Javadoc; the intro sentence was reworded to match.
• The Javadoc of OpenCensusMetricProducer.create() described a Resource parameter it has not taken since Stabilize MetricProducer, allow custom MetricReaders #5835; it now says the producer reads from the global OpenCensus MetricProducerManager. The @deprecated block is unchanged.
• Related: Stabilize MetricProducer, allow custom MetricReaders #5835, Remove deprecated metrics APIs #3990

Testing done

• Docs-only (README and one Javadoc sentence), test-exempt: no test added.
• ./gradlew :opencensus-shim:check passed (63 tests; spotless, ErrorProne/NullAway, checkstyle).
• The new snippet is the same call chain as OpenCensusMetricsTest#capturesOpenCensusAndOtelMetrics, which compiles and passes.
• No CHANGELOG entry (generated at release time); alpha artifact, no apidiff.

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Waiting on the author · refreshed 2026-09-12 12:50 UTC

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.27%. Comparing base (77b1ac4) to head (e1b4ee5).

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #8814      +/-   ##
============================================
- Coverage     91.28%   91.27%   -0.02%     
+ Complexity    10528    10526       -2     
============================================
  Files          1008     1008              
  Lines         28465    28465              
  Branches       3617     3617              
============================================
- Hits          25984    25981       -3     
  Misses         1685     1685              
- Partials        796      799       +3     

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