Emit tag filter list length

bin/agent-data-plane/src/components/tag_filterlist/mod.rs

@@ -169,14 +169,16 @@ impl TransformBuilder for TagFilterlistConfiguration {
 
     async fn build(&self, context: ComponentContext) -> Result<Box<dyn Transform + Send>, GenericError> {
         let metrics_builder = MetricsBuilder::from_component_context(&context);
+        let telemetry = Telemetry::new(&metrics_builder);
+        telemetry.set_rules_loaded(self.entries.len());
 
         Ok(Box::new(TagFilterlist {
             filters: compile_filters(&self.entries),
             configuration: self
                 .configuration
                 .clone()
                 .expect("configuration must be set via from_configuration"),
-            telemetry: Telemetry::new(&metrics_builder),
+            telemetry,
             context_cache: build_context_cache(),
         }))
     }
@@ -265,9 +267,11 @@ impl Transform for TagFilterlist {
                     None => break,
                 },
                 (_, new_entries) = watcher.changed::<Vec<MetricTagFilterEntry>>() => {
-                    self.filters = compile_filters(new_entries.as_deref().unwrap_or(&[]));
+                    let entries = new_entries.as_deref().unwrap_or(&[]);
+                    self.filters = compile_filters(entries);
                     self.context_cache = build_context_cache();
-                    debug!("Updated metric tag filterlist.");
+                    self.telemetry.set_rules_loaded(entries.len());
+                    debug!(rules_loaded = entries.len(), "Updated metric tag filterlist.");
                 },
             }
         }

bin/agent-data-plane/src/components/tag_filterlist/telemetry.rs

@@ -1,4 +1,4 @@
-use metrics::Counter;
+use metrics::{Counter, Gauge};
 use saluki_metrics::MetricsBuilder;
 
 use super::FilterMetricTagsOutcome;
@@ -10,17 +10,21 @@ pub struct Telemetry {
     noop_hits: Counter,
     metrics_modified: Counter,
     tags_filtered: Counter,
+    rules_loaded: Gauge,
 }
 
 impl Telemetry {
     pub fn new(builder: &MetricsBuilder) -> Self {
-        Self {
+        let me = Self {
             rule_hits: builder.register_debug_counter("tag_filterlist_rule_hits_total"),
             rule_misses: builder.register_debug_counter("tag_filterlist_rule_misses_total"),
             noop_hits: builder.register_debug_counter("tag_filterlist_noop_hits_total"),
             metrics_modified: builder.register_debug_counter("tag_filterlist_metrics_modified_total"),
             tags_filtered: builder.register_debug_counter("tag_filterlist_tags_filtered_total"),
-        }
+            rules_loaded: builder.register_debug_gauge("tag_filterlist_rules_loaded"),
+        };
+        me.rules_loaded.set(0.0);
+        me
     }
 
     pub fn record(&self, outcome: FilterMetricTagsOutcome) {
@@ -39,4 +43,11 @@ impl Telemetry {
             }
         }
     }
+
+    /// Records the number of tag filter rules currently loaded by the
+    /// transform. Called whenever the configuration is refreshed (initial
+    /// build or `metric_tag_filterlist` Remote Config update).
+    pub fn set_rules_loaded(&self, count: usize) {
+        self.rules_loaded.set(count as f64);
+    }
 }