Configurable base time unit for OTLP registry

[shakuzen]

We avoid making the base time unit configurable because it can lead to nasty issues if different applications are producing metrics data for the same meters in different units. However, it doesn't look hopeful open-telemetry/opentelemetry-collector-contrib#18903 is going to be resolved, and it's also not great that data from the OTLP registry through the Collector to Prometheus is going to have a different unit than everything else Prometheus scrapes (presumably, since Prometheus asks producers to use seconds for time).

We'll have to discuss this and what, if anything, it makes sense for us to do in Micrometer.

[zeitlinger]

I also ran into this issue today - and created a PR - based on the assumption that this is meeting the spec

[jonatan-ivanov]

For future reference, let me collect a couple of connected issues/prs:

• Semantic Conventions: default to seconds for duration units open-telemetry/opentelemetry-specification#2977
• Specify seconds unit for measuring durations open-telemetry/opentelemetry-specification#3388
• Mark "Instrumentation Units" and "Instrumentation Types" sections of the general metric semantic conventions as stable open-telemetry/opentelemetry-specification#3294
• Prometheus exporter does not convert time units to seconds open-telemetry/opentelemetry-collector-contrib#18903