Check group id, metric name, and group name uniqueness

[lquerel]

This PR detects duplications in the following:

• Group ID
• Metric name
• Group name (e.g., resource and event names)

Since there are existing duplications in the current registries (see the example below), we return warnings instead of errors. As usual, the --future flag must be used for future versions of the registry.

Example: https://github.com/search?q=repo%3Aopen-telemetry%2Fsemantic-conventions+path%3A%2F%5Emodel%5C%2F%2F+feature_flag&type=code

For now, the group name duplication detection is disabled.

Closes #306

[jsuereth]

@lquerel I think this is unblocked now if you can update the branch