Use seconds as default duration for FaaS metrics

[pyohannes]

This issue was created based on a discussion about #176.

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.

However, the following FaaS metrics still use milliseconds for measuring durations:

• faas.invoke_duration
• faas.init_duration
• faas.cpu.usage

Those metrics don't seem to be implemented currently, so changes can be done without a OTEL_SEMCONV_STABILITY_OPT_IN compatibility mechanism.