docs: Document QueryThrottler metrics

content/en/docs/24.0/reference/query-serving/metrics.md

@@ -12,6 +12,7 @@ These updates help users track query performance, identify costly execution plan
 Vitess now classifies query plans based on their execution strategies, allowing for more precise execution decisions.
 
 The classifications include:
+
 * `Local`: Queries executed locally on VTGate without involving any shard.
 * `Passthrough`: Queries forwarded to single shard without having any additional processing at VTGate.
 * `MultiShard`: Queries executed across multiple shards with controlled routing.
@@ -47,3 +48,66 @@ Some common optimizations include:
 
 Users can identify unoptimized queries using the plan classification in VTGate’s /debug/query_plans endpoint and apply the necessary improvements.
 By analyzing these metrics, users can fine-tune query execution, reduce latency, and improve overall performance in Vitess.
+
+## QueryThrottler Metrics
+
+The [QueryThrottler](../../features/query-throttler/) is a VTTablet component that evaluates queries to determine if they should be throttled based on configurable throttling strategies. VTTablet exposes several metrics to monitor [QueryThrottler](../../features/query-throttler/) activity.
+
+### Available Metrics
+
+VTTablet exposes the following metrics for monitoring [QueryThrottler](../../features/query-throttler/) activity:
+
+#### QueryThrottlerRequests
+
+Tracks the total number of requests evaluated by the query throttler.
+
+**Labels:**
+
+* `Strategy`: The throttling strategy being used (e.g., `TabletThrottler`)
+* `Workload`: The client application workload name from the `WORKLOAD_NAME` comment directive
+* `Priority`: The query priority value (0-100) determining query throttling preference (lower = higher priority)
+
+#### QueryThrottlerThrottled
+
+Tracks the number of requests that were throttled by the query throttler.
+
+**Labels:**
+
+* `Strategy`: The throttling strategy being used
+* `Workload`: The client application workload name from the `WORKLOAD_NAME` comment directive
+* `Priority`: The query priority value (0-100) determining query throttling preference (lower = higher priority)
+* `MetricName`: Name of the metric that triggered throttling (e.g., "cpu", "lag")
+* `MetricValue`: The value of the metric that triggered throttling
+* `DryRun`: Whether the throttler is in dry-run mode (true/false). In dry-run mode, throttling decisions are logged but queries are not throttled.
+
+#### QueryThrottlerTotalLatencyNs
+
+Measures the total latency in nanoseconds for each throttling request from entry to exit, including evaluation, metric checks, and all overhead.
+
+**Labels:**
+
+* `Strategy`: The throttling strategy being used
+* `Workload`: The client application workload name from the `WORKLOAD_NAME` comment directive
+* `Priority`: The query priority value (0-100) determining query throttling preference (lower = higher priority)
+
+#### QueryThrottlerEvaluateLatencyNs
+
+Measures the latency in nanoseconds of the strategy evaluation phase, specifically the time taken to make the throttling decision.
+
+**Labels:**
+
+* `Strategy`: The throttling strategy being used
+* `Workload`: The client application workload name from the `WORKLOAD_NAME` comment directive
+* `Priority`: The query priority value (0-100) determining query throttling preference (lower = higher priority)
+
+### Using QueryThrottler Metrics
+
+You can use these metrics to:
+
+* Monitor queries being evaluated for throttling
+* Track throttling rates by strategy, workload, and priority
+* Identify which metrics are triggering throttling decisions
+* Measure the performance overhead of throttling evaluation
+* Verify dry-run mode behavior before enabling throttling
+
+For example, you can use these metrics to determine if a particular workload or query priority is being throttled more frequently, or to measure the latency impact of throttling evaluation on query execution.