Implement internal metrics

[wsmoak]

Now that we have metrics... we can have metrics about the metrics! (and the logs and the spans.)

See Semantic conventions for OpenTelemetry SDK metrics

This document describes metrics emitted by the OpenTelemetry SDK components themselves about their internal state.

For example, otel.sdk.processor.log.queue.size - The number of log records in the queue of a given instance of an SDK log processor.

[wsmoak]

The Python SDK has the metrics instruments defined in opentelemetry-semantic-conventions/src/opentelemetry/semconv/_incubating/metrics/otel_metrics.py but nothing is reporting them yet.

Discussed in: https://cloud-native.slack.com/archives/C01PD4HUVBL/p1758901890444609

[kaylareopelle]

Great point, @wsmoak! Ruby has constants for those metrics defined in our latest semantic conventions package: https://github.com/open-telemetry/opentelemetry-ruby/blob/1f4f85ef19914fdb31cbd314c59f3f247b61df07/semantic_conventions/lib/opentelemetry/semconv/incubating/otel/metrics.rb#L130

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