Document legacy metrics

[atoulme]

Is your feature request related to a problem? Please describe.
otlp.exporter.seen and otlp.exporter.exported are currently emitted by the SDK but are not documented.

The expectation from reading the code is that those are considered legacy, and will replaced by semantic conventions-backed metrics.

Describe the solution you'd like
I would like to see a documentation of legacy metrics and their use case. I would like to understand if and when they will be considered for deprecation in favor of the semantic conventions ones.

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[jack-berg]

👍 Probably a section in https://opentelemetry.io/docs/languages/java/sdk/, and maybe https://opentelemetry.io/docs/languages/java/configuration/ that explains the context:

• The SDK start emitting internal metrics long before the otel conventions we're proposed. The old telemetry existed for so long it became defacto stable.
• We updated internal telemetry to support the new otel semantic conventions, and added configuration options to enable
• Programmatically, the pattern is to call setInternalTelemetryVersion, setMeterProvider(for example) to select the telemetry version you want, and to set the meter provider you want it recorded with
• With env var / system property config, set otel.experimental.sdk.telemetry.version to legacy or latest to configure the telemetry schema. The default is legacy, because of the defactor stable heritage previously mention.
• With declarative config, we want to be forward looking and long term have a default which uses the new semantic conventions. The property path is .instrumentation/development.java.otel_sdk.internal_telemetry_version. By default, internal telemetry is disabled. This affords us the ability to change the default to latest once semantic conventions are stabilized, without it being considered breaking.