doc: add FilterExec metrics to user-guide/metrics.md (#19043)

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part of #16602

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Start adding each operator's metrics to the user-guide page

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---------

Co-authored-by: Jeffrey Vo <[email protected]>
Co-authored-by: Martin Grigorov <[email protected]>

Author: 3 people

datafusion/physical-plan/src/filter.rs

@@ -686,10 +686,12 @@ struct FilterExecStream {
 
 /// The metrics for `FilterExec`
 struct FilterExecMetrics {
-    // Common metrics for most operators
+    /// Common metrics for most operators
     baseline_metrics: BaselineMetrics,
-    // Selectivity of the filter, calculated as output_rows / input_rows
+    /// Selectivity of the filter, calculated as output_rows / input_rows
     selectivity: RatioMetrics,
+    // Remember to update `docs/source/user-guide/metrics.md` when adding new metrics,
+    // or modifying metrics comments
 }
 
 impl FilterExecMetrics {

docs/source/user-guide/metrics.md

@@ -36,4 +36,12 @@ DataFusion operators expose runtime metrics so you can understand where time is
 
 ## Operator-specific Metrics
 
-TODO
+### FilterExec
+
+| Metric      | Description                                                       |
+| ----------- | ----------------------------------------------------------------- |
+| selectivity | Selectivity of the filter, calculated as output_rows / input_rows |
+
+## TODO
+
+Add metrics for the remaining operators