Define metric semantic conventions for database operations

[jgals]

What are you trying to achieve?
Defined metric semantic conventions for database operations.

What did you expect to see?
Database metric semantic convention documentation in markdown generated from YAML definitions as per open-telemetry/opentelemetry-specification#547.

Additional context.
These should be roughly analogous to the trace semantic conventions for database client calls defined here. For reference, metric semantic conventions for HTTP operations are already defined here.

[justinfoote]

I'll take this one.

[trask]

Closing this, as we have database connection metrics now, and there's another issue specifically for adding database operation metrics (#512).