enhancement(o11y): emit tag_filterlist_updates_total counter from TagFilterlist transform (#1818)

Adds a `tag_filterlist_updates_total` counter to the `TagFilterlist` transform so ADP reports how many times the `metric_tag_filterlist` configuration has been updated via Remote Config. The counter increments only on RC-triggered updates (not at build time), matching the behavior of `datadog.agent.tag_filterlist.updates` in the Core Agent. ADP surfaces this as `datadog.agent.tag_filterlist.updates` via the RAR remapper.

- [ ] `cargo test -p agent-data-plane tag_filterlist` — new `telemetry_records_updates` test plus all existing tests pass
- [ ] `cargo test -p agent-data-plane` — remapping output test and help text test pass

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: jesse.szwedko <jesse.szwedko@datadoghq.com>

Author: jszwedko

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

@@ -278,6 +278,7 @@ impl Transform for TagFilterlist {
                     self.filters = compile_filters(new_entries.as_deref().unwrap_or(&[]));
                     self.context_cache = build_context_cache();
                     self.telemetry.set_size(self.filters.len());
+                    self.telemetry.increment_updates();
                     debug!(rules_loaded = self.filters.len(), "Updated metric tag filterlist.");
                 },
             }
@@ -830,6 +831,23 @@ mod tests {
         assert_eq!(recorder.gauge("tag_filterlist_size"), Some(0.0));
     }
 
+    #[test]
+    fn telemetry_records_updates() {
+        let recorder = TestRecorder::default();
+        let _local = metrics::set_default_local_recorder(&recorder);
+
+        let builder = MetricsBuilder::default();
+        let telemetry = Telemetry::new(&builder);
+
+        assert_eq!(recorder.counter("tag_filterlist_updates_total"), Some(0));
+
+        telemetry.increment_updates();
+        assert_eq!(recorder.counter("tag_filterlist_updates_total"), Some(1));
+
+        telemetry.increment_updates();
+        assert_eq!(recorder.counter("tag_filterlist_updates_total"), Some(2));
+    }
+
     #[tokio::test]
     async fn dynamic_update_partial_replaces_filter() {
         let (cfg, sender) = ConfigurationLoader::for_tests(Some(serde_json::json!({})), None, true).await;

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

@@ -11,6 +11,7 @@ pub struct Telemetry {
     metrics_modified: Counter,
     tags_filtered: Counter,
     size: Gauge,
+    updates: Counter,
 }
 
 impl Telemetry {
@@ -22,6 +23,7 @@ impl Telemetry {
             metrics_modified: builder.register_debug_counter("tag_filterlist_metrics_modified_total"),
             tags_filtered: builder.register_debug_counter("tag_filterlist_tags_filtered_total"),
             size: builder.register_gauge("tag_filterlist_size"),
+            updates: builder.register_counter("tag_filterlist_updates_total"),
         }
     }
 
@@ -45,4 +47,8 @@ impl Telemetry {
     pub fn set_size(&self, count: usize) {
         self.size.set(count as f64);
     }
+
+    pub fn increment_updates(&self) {
+        self.updates.increment(1);
+    }
 }

bin/agent-data-plane/src/state/metrics/mod.rs

@@ -362,16 +362,11 @@ fn get_help_text(metric_name: &str) -> Option<&'static str> {
         "aggregator.dogstatsd_contexts" => Some("Count the number of dogstatsd contexts in the aggregator"),
         "aggregator.processed" => Some("Amount of metrics/services_checks/events processed by the aggregator"),
         "filterlist.size" => Some("Metric filter list size"),
-        "filterlist.updates" => {
-            Some("Incremented when a reconfiguration of the metric filterlist happened")
-        }
+        "filterlist.updates" => Some("Incremented when a reconfiguration of the metric filterlist happened"),
         "dogstatsd.listener_filtered_points" => Some("How many points were filtered out"),
-        "aggregator.dogstatsd_filtered_metrics" => {
-            Some("How many metrics were filtered in the time samplers")
-        }
-        "tag_filterlist.size" => {
-            Some("Tag filter list size")
-        },
+        "aggregator.dogstatsd_filtered_metrics" => Some("How many metrics were filtered in the time samplers"),
+        "tag_filterlist.size" => Some("Tag filter list size"),
+        "tag_filterlist.updates" => Some("Incremented when a reconfiguration of the tag filterlist happened"),
         "dogstatsd.processed" => Some("Count of service checks/events/metrics processed by dogstatsd"),
         "dogstatsd.packet_pool_get" => Some("Count of get done in the packet pool"),
         "dogstatsd.packet_pool_put" => Some("Count of put done in the packet pool"),
@@ -824,6 +819,10 @@ mod tests {
                 Context::from_static_parts("adp.tag_filterlist_size", &["component_id:dsd_tag_filterlist"]),
                 9.0,
             )),
+            Event::Metric(Metric::counter(
+                Context::from_static_parts("adp.tag_filterlist_updates_total", &["component_id:dsd_tag_filterlist"]),
+                11.0,
+            )),
         ];
 
         for metric in metrics {
@@ -839,6 +838,7 @@ mod tests {
         assert!(output.contains("dogstatsd__listener_filtered_points 5"));
         assert!(output.contains("aggregator__dogstatsd_filtered_metrics 7"));
         assert!(output.contains("tag_filterlist__size 9"));
+        assert!(output.contains("tag_filterlist__updates 11"));
         assert!(!output.contains("datadog__agent__filterlist__size"));
         assert!(!output.contains("datadog__agent__filterlist__updates"));
         assert!(!output.contains("datadog__agent__dogstatsd__listener_filtered_points"));
@@ -877,10 +877,7 @@ mod tests {
             get_help_text("aggregator.processed"),
             Some("Amount of metrics/services_checks/events processed by the aggregator")
         );
-        assert_eq!(
-            get_help_text("filterlist.size"),
-            Some("Metric filter list size")
-        );
+        assert_eq!(get_help_text("filterlist.size"), Some("Metric filter list size"));
         assert_eq!(
             get_help_text("filterlist.updates"),
             Some("Incremented when a reconfiguration of the metric filterlist happened")
@@ -894,5 +891,9 @@ mod tests {
             Some("How many metrics were filtered in the time samplers")
         );
         assert_eq!(get_help_text("tag_filterlist.size"), Some("Tag filter list size"));
+        assert_eq!(
+            get_help_text("tag_filterlist.updates"),
+            Some("Incremented when a reconfiguration of the tag filterlist happened")
+        );
     }
 }

bin/agent-data-plane/src/state/metrics/rules/dogstatsd.rs

@@ -28,6 +28,11 @@ pub fn get_dogstatsd_remappings() -> Vec<RemapperRule> {
             &["component_id:dsd_tag_filterlist"],
             "tag_filterlist.size",
         ),
+        RemapperRule::by_name_and_tags(
+            "adp.tag_filterlist_updates_total",
+            &["component_id:dsd_tag_filterlist"],
+            "tag_filterlist.updates",
+        ),
         RemapperRule::by_name_and_tags(
             "adp.object_pool_acquired",
             &["pool_name:dsd_packet_bufs"],