[grafana] add description panel to dashboards

[puckpuck]

Changes

Fixes #1771

This adds a description panel to the top of each dashboard. The panel includes a short description of the dashboard and a note that it may take 5 minutes for the panels to start rendering data after the demo starts.

2 dashboard panels were also fixed:

1. Quote Service batch span processor on the Demo dashboard was referencing an incorrect tag value
2. Top 7 Highest Endpoint Latencies Mean Over Range on the spanmetrics dashboard was referencing an incorrect metric name

Merge Requirements

For new features contributions, please make sure you have completed the following
essential items:

• CHANGELOG.md updated to document new feature additions
• Appropriate documentation updates in the docs
• Appropriate Helm chart updates in the helm-charts

Maintainers will not merge until the above have been completed. If you're unsure
which docs need to be changed ping the
@open-telemetry/demo-approvers.

[mviitane]

Nice UX improvement! Added few minor comments. The dashboards were working fine.