Use seconds as default duration for RPC 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:

• rpc.server.duration
• rpc.client.duration

For consistency, those histograms should be changed to use seconds. Those metrics are already in use, and to avoid unnecessary disruptions the change should happen during stabilization of RPC semantic conventions and should be covered by the OTEL_SEMCONV_STABILITY_OPT_IN mechanism.