Prometheus exporter: Unit in metric names

[dashpole]

From the OTel prometheus spec:

The unit MUST be added as a suffix to the metric name, and SHOULD be converted to base units recommended by OpenMetrics when possible. The unit suffix comes before any type-specific suffixes.

I would recommend against converting to base units (e.g. converting milliseconds to seconds), but would recommend converting abbreviations to full words for commonly-used units (e.g. ms to milliseconds).

As an example of what metrics should look like when this is complete, a metric with name foo, and unit ms should have the resulting name: foo_milliseconds.