Don't recommend converting to prometheus base units, but do convert to full words

[dashpole]

As described in #2497 (comment) (and the following discussion), it isn't possible to convert from units to base units (e.g. convert milliseconds to seconds) for all metric types. This PR withdraws that recommendation.

I opened a few issues for adding units to metric names:

• Prometheus exporter: Unit as suffix for metric name opentelemetry-swift#359
• Prometheus exporter: Unit in metric names opentelemetry-rust#927
• Prometheus exporter: unit, name, label sanitization improvements opentelemetry-dotnet#3975
• Prometheus exporter: Add unit to metric names, and convert unit to words opentelemetry-cpp#1840
• Prometheus exporter: Add unit to metric names, and convert unit to words opentelemetry-js#3466

During that process, I didn't want to recommend converting to base units for the reasons above, but I do still think implementations should convert to full words where possible.

cc @gouthamve @jmacd

[gouthamve]

I don't understand why we can't convert from milliseconds to seconds on fixed bucket histograms. Its easy.

Its not possible in exponential histograms, and we have a pending issue here: #2977

I would wait for #2977 to resolve before we decide this tbh.

And even if we keep milliseconds in exponential histograms, I might be tempted to keep seconds for fixed bucket histograms. I need to think more about this.

[dashpole]

Discussed this as the prometheus wg. We are going to hold off on this while we discuss #2977