Prometheus exporter metric names should include unit suffixes.

[dashpole]

Spec: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/data-model.md#metric-metadata-1

The Unit of an OTLP metric point MUST be added as OpenMetrics UNIT metadata. Additionally, 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.

The things we need to do are:

• Map OTel units to "base" units. Base units include seconds, bytes, joules, grams, meters, ratios, volts, amperes, and celsius.
• Add the unit as a suffix to the metric name.