BREAKING: Use seconds as default duration for FaaS duration histograms

[pyohannes]

Fixes #382

Changes

General guidelines for metric instrumentation units require that for measuring durations, seconds should be used:

When instruments are measuring durations, seconds (i.e. s) SHOULD be used.

This PR brings the FaaS metrics faas.invoke_duration, faas.init_duration, and faas.cpu.usage in line with this requirement.

As I couldn't find a place where those metrics are currently implemented or used, this change is proposed without recourse to any compatibility mechanism.

Merge requirement checklist

• CONTRIBUTING.md guidelines followed.
• CHANGELOG.md updated for non-trivial changes.
• schema-next.yaml updated with changes to existing conventions.

[jsuereth]

Approving this under the assumption that these semconv aren't used heavily and this breaking change won't actually affect any (or not many) users.

[jsuereth]

@pyohannes Can you open a bug against the specification asking for SCHEMA files to allow us to denote BREAKING changes where a metric changes its unit?

EDIT: Also, FYI - I check your schema_url mark. When Schema URL cannot express a breaking change, at this point we should open a collection of bugs for schema-url improvements.

[pyohannes]

@jsuereth

Can you open a bug against the specification asking for SCHEMA files to allow us to denote BREAKING changes where a metric changes its unit?

• Schema transformations should be able to convert metric units opentelemetry-specification#3729