feat(config): add pull metric reader support to declarative config (#5216)

* add pull metric reader support to declarative config

Implements PullMetricReader and PullMetricExporter support in
declarative file configuration:

- _create_prometheus_metric_reader() — dynamically imports
  PrometheusMetricReader, maps config fields (host, port,
  without_target_info_development), and starts HTTP server
- _create_pull_metric_exporter() — dispatches to prometheus or
  loads plugin readers via opentelemetry_pull_metric_exporter
  entry point group
- _create_pull_metric_reader() — handles producers (warns, #5074)
  and cardinality_limits (warns), delegates to exporter factory

producers and cardinality_limits log warnings as not yet supported.

Assisted-by: Claude Opus 4.6

* rename changelog fragment to PR #5216

* fix pylint R1726: simplify boolean condition

Remove redundant 'or False' from bool() conversion.

Assisted-by: Claude Opus 4.6

* fix pylint R6301: use self in prometheus test methods

Add provider assertions so test methods use self, satisfying
pylint's no-self-use check.

Assisted-by: Claude Opus 4.6

* address review: add type annotation, validate entry point group in test

- Add PrometheusMetricExporterConfig type annotation to
  _create_prometheus_metric_reader config parameter
- Assert entry_points is called with the correct group name
  (opentelemetry_pull_metric_exporter) in plugin loading test

Assisted-by: Claude Opus 4.6

* split pull metric reader tests into separate class to fix pylint R0904

* fix test class split: move push exporter tests to separate class

The merge brought in push exporter plugin tests from #5128 which
ended up in TestCreatePullMetricReaders. Moved them to a new
TestCreateMetricReadersGeneral class with the _make_periodic_config
helper they need.

Assisted-by: Claude Opus 4.6

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Co-authored-by: Lukas Hering <40302054+herin049@users.noreply.github.com>