Exemplar support

[emschwartz]

Looks like this might be blocked waiting on upstream support:

• Metrics: Add support for exemplars open-telemetry/opentelemetry-python#2407
• Exemplar support prometheus/client_python#648

[actualwitch]

This PR partially resolves the issue - for now exemplars are only supported in prometheus client, but for people who enable that backend for autometrics-py we expose a AUTOMETRICS_EXEMPLARS env var you can use to enable them in autometrics itself.

[emschwartz]

@actualwitch I think we can consider this done for now and potentially open another issue for later about supporting exemplars with OpenTelemetry whenever they add support for the feature upstream