SDK spec does not reference self-observability metrics defined in semantic conventions

[cijothomas]

The OpenTelemetry SDK specification does not currently require, recommend, or even acknowledge that SDKs emit self-observability ("internal") metrics.

Meanwhile, the semantic-conventions repo defines a full set of such metrics in docs/otel/sdk-metrics.md (processors, exporters, metric readers, etc.), all in development status. There is no link between the two: the spec doesn't point at semantic conventions for these metrics, and semantic conventions has no spec mandate to anchor to.

Proposal: the SDK spec should add a short section pointing implementers at the semantic conventions as the source of truth for SDK self-observability metric names, attributes, and values.

This linkage is a prerequisite for any normative discussion of the contents of those metrics (e.g., which error.type values processors should report).

[cijothomas]

While implementing internal metric for OTel Rust (open-telemetry/opentelemetry-rust#3514), I noticed some issues need broader discussion in the spec, and also in the semantic-convention.
eg: https://github.com/open-telemetry/semantic-conventions/pull/3710/changes

First step would be to add a link between spec and semantic convention. (I'll send a PR to make this happen)