Clear metric documentation

When adding, modifying, or deprecating metrics, ensure comprehensive and clear documentation. Include:

1. Descriptive names for new metrics that reflect their purpose
2. Clear explanations of what each metric measures
3. The dimensions/labels available for the metric
4. Explicit mention of any deprecated metrics and their replacements

For example, when documenting a new metric:

### VTGate Metrics

Added two new metrics with query type, plan type and tablet type as dimensions:
1. QueryProcessed - This counts the number of queries processed
2. QueryRouted - This counts the number of shards the query was executed on

Deprecated:
1. QueriesProcessed - Replaced by QueryProcessed
2. TableMetrics - Replaced by QueryTables

This practice ensures users can effectively monitor their systems, understand metric changes, and maintain accurate dashboards and alerts as your observability tools evolve.