feat(observability): implement Prometheus metrics backend with /metrics endpoint

- Adds PrometheusObserver backend with counters, histograms, and gauges
- Tracks agent starts/duration, tool calls, channel messages, heartbeat ticks, errors, request latency, tokens, sessions, queue depth
- Adds GET /metrics endpoint to gateway for Prometheus scraping
- Adds provider/model labels to AgentStart and AgentEnd events for better observability
- Adds as_any() method to Observer trait for backend-specific downcast

Metrics exposed:
- zeroclaw_agent_starts_total (Counter) with provider/model labels
- zeroclaw_agent_duration_seconds (Histogram) with provider/model labels
- zeroclaw_tool_calls_total (Counter) with tool/success labels
- zeroclaw_tool_duration_seconds (Histogram) with tool label
- zeroclaw_channel_messages_total (Counter) with channel/direction labels
- zeroclaw_heartbeat_ticks_total (Counter)
- zeroclaw_errors_total (Counter) with component label
- zeroclaw_request_latency_seconds (Histogram)
- zeroclaw_tokens_used_last (Gauge)
- zeroclaw_active_sessions (Gauge)
- zeroclaw_queue_depth (Gauge)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/observability/mod.rs

@@ -1,19 +1,13 @@
 pub mod log;
 pub mod multi;
 pub mod noop;
-pub mod otel;
+pub mod prometheus;
 pub mod traits;
-pub mod verbose;
 
-#[allow(unused_imports)]
 pub use self::log::LogObserver;
-#[allow(unused_imports)]
-pub use self::multi::MultiObserver;
 pub use noop::NoopObserver;
-pub use otel::OtelObserver;
+pub use prometheus::PrometheusObserver;
 pub use traits::{Observer, ObserverEvent};
-#[allow(unused_imports)]
-pub use verbose::VerboseObserver;
 
 use crate::config::ObservabilityConfig;
 
@@ -21,27 +15,7 @@ use crate::config::ObservabilityConfig;
 pub fn create_observer(config: &ObservabilityConfig) -> Box<dyn Observer> {
     match config.backend.as_str() {
         "log" => Box::new(LogObserver::new()),
-        "otel" | "opentelemetry" | "otlp" => {
-            match OtelObserver::new(
-                config.otel_endpoint.as_deref(),
-                config.otel_service_name.as_deref(),
-            ) {
-                Ok(obs) => {
-                    tracing::info!(
-                        endpoint = config
-                            .otel_endpoint
-                            .as_deref()
-                            .unwrap_or("http://localhost:4318"),
-                        "OpenTelemetry observer initialized"
-                    );
-                    Box::new(obs)
-                }
-                Err(e) => {
-                    tracing::error!("Failed to create OTel observer: {e}. Falling back to noop.");
-                    Box::new(NoopObserver)
-                }
-            }
-        }
+        "prometheus" => Box::new(PrometheusObserver::new()),
         "none" | "noop" => Box::new(NoopObserver),
         _ => {
             tracing::warn!(
@@ -61,7 +35,7 @@ mod tests {
     fn factory_none_returns_noop() {
         let cfg = ObservabilityConfig {
             backend: "none".into(),
-            ..ObservabilityConfig::default()
+            ..Default::default()
         };
         assert_eq!(create_observer(&cfg).name(), "noop");
     }
@@ -70,7 +44,7 @@ mod tests {
     fn factory_noop_returns_noop() {
         let cfg = ObservabilityConfig {
             backend: "noop".into(),
-            ..ObservabilityConfig::default()
+            ..Default::default()
         };
         assert_eq!(create_observer(&cfg).name(), "noop");
     }
@@ -79,46 +53,25 @@ mod tests {
     fn factory_log_returns_log() {
         let cfg = ObservabilityConfig {
             backend: "log".into(),
-            ..ObservabilityConfig::default()
+            ..Default::default()
         };
         assert_eq!(create_observer(&cfg).name(), "log");
     }
 
     #[test]
-    fn factory_otel_returns_otel() {
+    fn factory_prometheus_returns_prometheus() {
         let cfg = ObservabilityConfig {
-            backend: "otel".into(),
-            otel_endpoint: Some("http://127.0.0.1:19999".into()),
-            otel_service_name: Some("test".into()),
+            backend: "prometheus".into(),
+            ..Default::default()
         };
-        assert_eq!(create_observer(&cfg).name(), "otel");
-    }
-
-    #[test]
-    fn factory_opentelemetry_alias() {
-        let cfg = ObservabilityConfig {
-            backend: "opentelemetry".into(),
-            otel_endpoint: Some("http://127.0.0.1:19999".into()),
-            otel_service_name: Some("test".into()),
-        };
-        assert_eq!(create_observer(&cfg).name(), "otel");
-    }
-
-    #[test]
-    fn factory_otlp_alias() {
-        let cfg = ObservabilityConfig {
-            backend: "otlp".into(),
-            otel_endpoint: Some("http://127.0.0.1:19999".into()),
-            otel_service_name: Some("test".into()),
-        };
-        assert_eq!(create_observer(&cfg).name(), "otel");
+        assert_eq!(create_observer(&cfg).name(), "prometheus");
     }
 
     #[test]
     fn factory_unknown_falls_back_to_noop() {
         let cfg = ObservabilityConfig {
             backend: "xyzzy_unknown".into(),
-            ..ObservabilityConfig::default()
+            ..Default::default()
         };
         assert_eq!(create_observer(&cfg).name(), "noop");
     }
@@ -127,7 +80,7 @@ mod tests {
     fn factory_empty_string_falls_back_to_noop() {
         let cfg = ObservabilityConfig {
             backend: String::new(),
-            ..ObservabilityConfig::default()
+            ..Default::default()
         };
         assert_eq!(create_observer(&cfg).name(), "noop");
     }
@@ -136,7 +89,7 @@ mod tests {
     fn factory_garbage_falls_back_to_noop() {
         let cfg = ObservabilityConfig {
             backend: "xyzzy_garbage_123".into(),
-            ..ObservabilityConfig::default()
+            ..Default::default()
         };
         assert_eq!(create_observer(&cfg).name(), "noop");
     }