clarify pool name being unique

[maryliag]

Changes

Working on the prototype for DB Metrics, the recommendation for using just host:port as a fallback for pool name was not enough to identify a pool, so this PR adds example of how to make the name more unique.

Merge requirement checklist

• CONTRIBUTING.md guidelines followed.
• 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