SemConv guidance: document how to define metrics

[lmolkova]

See https://github.com/open-telemetry/semantic-conventions/blob/main/docs/general/how-to-define-semantic-conventions.md#defining-metrics

Some areas to cover:

• granularity / when to define a new metric vs reuse the same metric definition across multiple operations
• cardinality
• duration histogram covers request rate/error rate
• errors: link to recording errors

[lmolkova]

Related: #1876

[thompson-tomo]

We should also cover instrumentation type selection counter vs gauge. Another area would be units ie seconds for time spans.

[lmolkova]

#78 - document duration

[lmolkova]

#722 - mention that adding attributes is usually breaking