Generate semantic conventions for metrics.

[lalitb]

As discussed here:

https://github.com/open-telemetry/opentelemetry-rust/tree/main/opentelemetry-semantic-conventions/scripts/templates we don't have metric attributes generated? is that something we can add as well?

Yes, there were some docker errors while generating the source files for metrics. So thought of publishing them as separate PR.

Originally posted by @lalitb in https://github.com/open-telemetry/opentelemetry-rust/issues/1596#issuecomment-1978034722