SemConv guidance: document how to define spans

[lmolkova]

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

Some areas to cover:

• granularity / when to define a new span vs reuse the same span definition across multiple operations
• when to report span vs event
• span name pattern
• *operation.name attributes
• sampling-relevant attributes
• span duration vs metric duration
• span status: link to recording errors
• describing groups of spans (parent vs link)