ExplicitBucketHistogram resets counts to zero even if aggregation temporality is cumulative

[ocelotl]

          While this might help for deltas, the SDK should be returning metrics every single collection for readers that are CUMULATIVE (the default). However that seems to not be working corrctly https://github.com/open-telemetry/opentelemetry-python/issues/3277#issuecomment-1684521139

Yes, noticed that now that I added another test case. I think this is a problem for the explicit bucket histogram aggregation who is resetting to zero all bucket counts after every collection even in the aggregation temporality is cumulative. I say we review this PR and address that issue separately.

Originally posted by @aabmass in #3335 (review)

[aabmass]

I looked into this a bit and found the issue #3277 (comment)

I think I found the issue

opentelemetry-python/opentelemetry-sdk/src/opentelemetry/sdk/metrics/_internal/aggregation.py

Lines 292 to 293 in d8490c5

	if not any(self._bucket_counts):
	return None

This short circuiting logic is incorrect, it just assumes if no new counts were recorded since the last collection that we can return None. We should return the previous point values instead.