prometheus exporter: use explicit histograms

[isaaczinda]

The current prometheus exporter can't export histograms. This is because the current converter passes the prometheus exporter exponential histograms, which it doesn't know how to accept. This PR teaches the converter to produce explicit histograms instead, which the prometheus exporter does know how to accept.

Testing:

• a simple integration test to makes sure the /metrics endpoint can produce is exposing histogram points
• fairly involved unit tests of the converter from lightstep histogram to OTel explicit histogram

R: @jmacd