Db metrics pending requests

[maryliag]

Changes

During the prototype for the DB Metrics, found that is not always possible to have a cumulative value. Having just a current value makes more sense. Existing implementations of this metric are already using the current or are using a wrong implementation of the cumulative.

Merge requirement checklist

• [x] CONTRIBUTING.md guidelines followed.
• [x] Change log entry added, according to the guidelines in When to add a changelog entry.
  • If your PR does not need a change log, start the PR title with [chore]

[maryliag]

cc @open-telemetry/semconv-db-approvers