Write Metric guidance that description should be a full sentance.

[reyang]

OpenMetrics provided such example: https://github.com/OpenObservability/OpenMetrics/blob/main/specification/OpenMetrics.md#overall-structure

The current metrics semantic convention is different (and inconsistent):

Ask:

1. Make it consistent at least for the semantic conventions that are in scope for the initial stable release.
2. Align with OpenMetrics, description should have the first letter uppercase, and end with ..

Additional Ask (not blocking):

1. The build tool should fail/block the CI if there is any description not following the convention/rule.