Return proxy instruments from ProxyMeter

[aabmass]

Fixes #2144

This PR implements proxy instruments to be returned from the metrics API before an SDK is set up. This is slightly different than how this is implemented for tracing because of the complication of asynchronous instruments. In tracing, we resolve the proxied Tracer whenever spans are created by a user:

opentelemetry-python/opentelemetry-api/src/opentelemetry/trace/__init__.py

Lines 411 to 415 in 18b5cb0

	def start_span(self, *args, **kwargs) -> Span: # type: ignore
	return self._tracer.start_span(*args, **kwargs) # type: ignore
	def start_as_current_span(self, *args, **kwargs) -> Span: # type: ignore
	return self._tracer.start_as_current_span(*args, **kwargs) # type: ignore

This would work for synchronous instruments. However, the user never interacts with asynchronous instruments, so we need to keep them reachable by reference so when an SDK is swapped in, we can create SDK instances of each async instrument.

[ocelotl]

Suggested change

	if self._real_meter_provider:
	if self._real_meter_provider is not None:

[aabmass]

These should be equivalent here right? I'm happy to change it if this is a general thing we enforce throughout our code base.

[ocelotl]

Most of the time it is not a big deal, but it can be. It is part of PEP8 because of this reason:

Also, beware of writing if x when you really mean if x is not None -- e.g. when testing whether a variable or argument that defaults to None was set to some other value. The other value might have a type (such as a container) that could be false in a boolean context!

I don't think we have been consistent on enforcing this, though.

[lzchen]

Should wait on #2182 to be merged before this.

[aabmass]

Should be good to go now 👍

[ocelotl]

Should be good to go now +1

Hm, looks like tests are still failing...

[aabmass]

@ocelotl failures are in contrib builds related to the release

ERROR: pip's dependency resolver does not currently take into account all the packages that are installed. This behaviour is the source of the following dependency conflicts.
opentelemetry-sdk 1.5.0 requires opentelemetry-api==1.5.0, but you have opentelemetry-api 1.6.0 which is incompatible.
opentelemetry-sdk 1.5.0 requires opentelemetry-semantic-conventions==0.24b0, but you have opentelemetry-semantic-conventions 0.25b0 which is incompatible.
Successfully installed opentelemetry-semantic-conventions-0.25b0

I think #2199 should fix it?