Make processor queue capacity metrics observable

[cijothomas]

Description

Report span and log processor queue capacity with observable up-down counters instead of recording the capacity once with synchronous counters.

A processor can be constructed before the SDK installs the real meter provider. In that case, the one-time synchronous measurement is made against a proxy instrument and is not replayed when the provider is installed, so queue capacity is never exported. Observing the fixed capacity during collection avoids that initialization-order dependency and remains compliant with the metrics semantic conventions.

Adds regression coverage for processors created before the real meter provider is installed.

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Waiting on reviewers · refreshed 2026-09-04 17:28 UTC

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[cijothomas]

@DylanRussell Please review. Hit the issue while working on open-telemetry/opentelemetry-demo#3932 and it worked around by re-ordering, but this PR would be a better fix.