Prometheus exporter: unit, name, label sanitization improvements

[dashpole]

A few thing I found during a review of the current implementation:

• Name should match [a-zA-Z_:]([a-zA-Z0-9_:])*. From the OTel prometheus spec on metric names, the metric name MUST match [a-zA-Z_:]([a-zA-Z0-9_:])*. The current implementation only sanitizes . and -. !@#$% characters, for example, should be replaced with _.
• Metric names must not start with a number. From the OpenMetrics ABNF](https://github.com/OpenObservability/OpenMetrics/blob/main/specification/OpenMetrics.md#abnf), the initial character excludes numbers. The recommended solution is to add a prefix the name if the first character is a number.
• From the OTel Prometheus spec, Double _ characters in metric names should be replace with a single _.
• The OTel unit should be converted to the full word when possible. For example, ms should be converted to milliseconds.