[SDK] Support MeterConfigurator updates and example

[adityabavadekar]

Fixes #4256

Adds the ability to update a MeterConfigurator (and the MeterConfig of
existing meters) at runtime. Following the pattern used in #4065 and #4224.

This implements the optional requirement to allow updating the
MeterProvider's MeterConfigurator:
https://opentelemetry.io/docs/specs/otel/metrics/sdk/#meterconfigurator

This function is called when a Meter is first created, and for each
outstanding Meter when a MeterProvider's MeterConfigurator is updated
(if updating is supported).

Changes

• Adds UpdateMeterConfigurator to MeterProvider to support setting new
  meter configs at runtime. The update holds the same lock as GetMeter, so a
  meter is never returned with a config that is out of date with the provider
  configurator. A null configurator is ignored.
• Moves the Meter enabled flag into an atomic state shared with its
  instruments, so updates reach existing instruments without blocking
  instrument creation or Collect.
• Adds MeterContext::SetMeterConfigurator.
• Adds tests and an example.

Behavior

While a Meter is disabled (also covered in the example README):

• Existing instruments don't need to be recreated. They start recording again once the Meter is enabled.
• New measurements are dropped and async callbacks are not called.
• Data recorded before the Meter was disabled is still exported.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: adityabavadekar / name: Aditya Bavadekar (0209a3d)

[codecov]

Codecov Report

❌ Patch coverage is 76.71233% with 34 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.63%. Comparing base (cdd17eb) to head (86b8516).

Files with missing lines	Patch %	Lines
sdk/src/metrics/sync_instruments.cc	68.00%	32 Missing ⚠️
sdk/src/metrics/meter.cc	90.91%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4313      +/-   ##
==========================================
- Coverage   86.81%   86.63%   -0.18%     
==========================================
  Files         525      526       +1     
  Lines       20736    20809      +73     
==========================================
+ Hits        18000    18025      +25     
- Misses       2736     2784      +48     

Files with missing lines	Coverage Δ
...etry/sdk/instrumentationscope/scope_configurator.h	100.00% <ø> (ø)
sdk/include/opentelemetry/sdk/metrics/meter.h	62.50% <100.00%> (+5.36%)	⬆️
...de/opentelemetry/sdk/metrics/meter_enabled_state.h	100.00% <100.00%> (ø)
...clude/opentelemetry/sdk/metrics/sync_instruments.h	100.00% <100.00%> (ø)
sdk/src/metrics/meter_context.cc	94.12% <100.00%> (+0.31%)	⬆️
sdk/src/metrics/meter_context_factory.cc	100.00% <ø> (ø)
sdk/src/metrics/meter_provider.cc	93.69% <100.00%> (+0.83%)	⬆️
sdk/src/metrics/meter_provider_factory.cc	100.00% <ø> (ø)
sdk/src/metrics/meter.cc	84.00% <90.91%> (+1.19%)	⬆️
sdk/src/metrics/sync_instruments.cc	65.46% <68.00%> (-1.98%)	⬇️

... and 3 files with indirect coverage changes

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[dbarker]

Thanks for the feature and tests! Please see some initial comments below.

[ThomsonTan]

for (auto &meter : context_->GetMeters())GetMeters() is explicitly non-thread-safe, so this loop is safe only under a cross-class invariant: every production mutation of meters_ currently reaches AddMeter() or RemoveMeter() through a MeterProvider method that holds lock_. That invariant is not stated here, and it is not structurally enforced because MeterContext exposes both mutators publicly.

T1: provider lock_ -> GetMeters() -> iterate borrowed span
T2: meter_lock_    -> AddMeter()  -> meters_ may reallocate

lock_ does not exclude a direct/future T2 caller. The analogous tracer and logger loops explicitly document their ownership invariant, and their collections are provider-private.

Please either enforce this invariant by using ForEachMeter() or making the mutators provider-only, or at minimum add the equivalent ownership comment above this loop so future callers do not introduce a race.

[adityabavadekar]

Switched to ForEachMeter

[dbarker]

Thanks for the updates! Please see some feedback and requested changes below.

[dbarker]

Hi @adityabavadekar, Just checking in. Are you still interested in contributing this PR?

[adityabavadekar]

Yes, I am still interested, and I have fixed the issues you mentioned. Please review.
The changes:

• Reverted the IsEnabled() warning conditions, at 4 sites rather than 2 since
  WarnOnDuplicateInstrument is also identity-based. All 4 match main again.
• Made meter_enabled_state required, updating 27 test construction sites, and removed 2 null
  checks it makes unreachable.
• Removed the deadlock test.
• Fixed a bug introduced earlier: SyncMetricStorage::Bind returning nullptr was wrapped into
  a handle with null storage, so instruments bound while disabled stayed silent after
  re-enabling. It now binds lazily on enable. BoundCounterObservesMeterEnabledState masked
  this by using two separate states.
• Merged main and fixed the iwyu diagnostics.

[marcalff]

Trying to resolve remaining IWYU issues, but doing that with the web UI with comments ... it may take a few attempts.

[marcalff]

@adityabavadekar

Thanks for the contribution.

I did small fixes to includes, to make the IWYU CI pass in all 3 flavors, so CI is now clean.

@ThomsonTan

Please look again, I think your comments have been addressed.

[adityabavadekar]

@dbarker I have fix the GCC build issue and all of the tests are passing now. I've also updated the PR description to match latest changes. Please let me know if anything else is needed before this can be merged.
Thanks!