Generate constants for metric definitions

[trask]

Related to open-telemetry/opentelemetry-java-instrumentation#9478

Another option is to provide an API that creates the metric using these constants internally, though that doesn't help us in library instrumentation for all of the unstable metrics at least where we don't want to have a dependency on unstable semconv artifact, but we still want the constants for verification in tests.

[jack-berg]

I'm very supportive of this idea! Some thoughts:

• I think we should only generate incubating constants to start (but include stable metrics in the incubating files) to start.
• In this proposal we generate metric name, unit description. Its still possible to misuse the conventions by choosing the wrong instrument type. Have you considered generating functions which return fully resolved metrics? I.e. something like:

public static DoubleHistogram httpServerRequestDurationMetric(Meter) { ... }

We could of course do both. Generating the name, unit, description constants is more flexible in the sense that it allows you to more easily create metrics matching a particular metric convention's name and description, but with a custom unit. I.e. http.server.request.duration with ms unit instead of the default s. But this greases the tracks to not conform to the conventions, which I'd argue we ought not to do.