Support OpenMetrics 2.0 exposition format

[rbizos]

OpenMetrics 2.0 is now available (experimental): https://prometheus.io/docs/specs/om/open_metrics_spec_2_0/

Prometheus common have it implemented prometheus/common#893
A PR is on the way for client_golang prometheus/client_golang#2132

It would be nice for client_python to support exposing OpenMetrics 2.0 as well.

Proposed approach, mirroring what client_golang:

• Keep a single existing opt-in switch for OpenMetrics exposition rather than adding a second flag specifically for 2.0.
• Once OpenMetrics exposition is opted into, rely purely on content neciation.

Related work:

• Allow native histograms emission only when OM 2.0 is requested #1122
• Allow users to create native histograms #1104
• OM text exposition for NH #1087