Fix OpenMetrics valid label keys, and specify prometheus conversion for metric name.

[dashpole]

Changes

This fixes an incorrect statement about the allowed characters in prometheus label keys found here: open-telemetry/opentelemetry-go#3135 (comment).

During the process, I also realized we don't specify conversion rules for the metric name, including the rules for sanitizing the metric name when converting from OTLP to Prom.

Source: https://github.com/OpenObservability/OpenMetrics/blob/main/specification/OpenMetrics.md#abnf

In that spec, metricname-initial-char allows :, but label-name-initial-char does not.

[dashpole]

cc @open-telemetry/wg-prometheus

[jmacd]

@dashpole related question, maybe not answered in this PR:
What is prescribed for empty label keys?
The specification merely requires valid Unicode sequences. I think I would prefer it said non-empty sequences, but I'm not sure what SDKs should do in this case. What do you think?

[dashpole]

For Prometheus/OpenMetrics, empty label keys aren't allowed. We should drop + warn for an empty label. For exporters written using prometheus client libraries, that is already done by the client's validation (e.g. prometheus/client_golang). It might be good to include a catchall statement somewhere to the effect of: "exporters should produce valid prometheus/openmetrics metrics, or MUST drop the metric and emit a warning".