[Semantic Conventions] Http metric semantic conventions don't use semantic convention for defining units.

[jsuereth]

HTTP semantic conventions define units like "microseconds". According to this, the unit needs to be on ucum

[pichlermarc]

This seems to be related to #705 and #1794.

The same seems to be the case for

• FaaS Semantic Conventions
• http Semantic Conventions
• RPC Semantic Conventions

while System Metrics Semantic Conventions seem to do it as the General Metric Semantic Conventions suggest.

It seems like changing this in the conventions I listed above would just be an editorial change. I'd be interested in picking this up.

Edit: typo