Verify prev point is returned for cumulative readers instead of None

[srikanthccv]

This LGTM. For cumulative readers though, we need to make sure they return the previous point value instead of None. For example, with a prometheus exporter:

counter.add(10, {"foo": "bar"})
# gets scraped, reports 10 for {foo="bar"}
# ... no measurements

# gets scraped again, should continue to report 10 for {foo="bar"}
# even though there were no measurements.

Do you know if that is the case? If not we should open another issue for rc2.

Originally posted by @aabmass in #2745 (review)

[ocelotl]

I am a bit confused, what is a "cumulative reader"? Is it an arbitrary designation or can we find out if a MetricReader is cumulative from looking into a certain characteristic of the MetricReader?