Clarify metric view measurement processing

[danielgblanco]

Fixes #3829

Changes

• Adds clarification on measurement processing for metric views and their relation the resulting metric streams, particularly when multiple views match the same instrument
• Fixes example that should generate a warning of conflicting metric identities
• Adds example for drop aggregation as a non-conflicting metric view

• Related issues #
• Related OTEP(s) #
• Links to the prototypes (when adding or changing features)
• CHANGELOG.md file updated for non-trivial changes
• spec-compliance-matrix.md updated if necessary

[reyang]

LGTM, thanks @danielgblanco!

[cijothomas]

Very good examples to clarify!

[danielgblanco]

Thanks both. I don't have write access on this repo so feel free to merge whenever you think it's good to go :)

[reyang]

Thanks both. I don't have write access on this repo so feel free to merge whenever you think it's good to go :)

FYI, we follow this process https://github.com/open-telemetry/opentelemetry-specification/blob/main/CONTRIBUTING.md#how-to-get-your-pr-merged.