Test that metrics methods can be called concurrently

[ocelotl]

Originally posted by @codeboten in https://github.com/open-telemetry/opentelemetry-python/pull/1887#discussion_r710558412

This may help.

[aabmass]

@ocelotl I'll pick this up like we discussed. Some specifics are that we need to add locks around the global meter provider and potentially some other spots. Also should document concurrency guarantees in the docstrings.