feat: add OpenTelemetry tracing and metrics observer

* feat: add OpenTelemetry tracing and metrics observer

Add OtelObserver that exports traces and metrics via OTLP HTTP/protobuf
to any OpenTelemetry-compatible collector (Jaeger, Grafana Tempo, etc.).

- ObserverEvents map to OTel spans (AgentEnd, ToolCall, Error) and
  metric counters (AgentStart, ChannelMessage, HeartbeatTick)
- ObserverMetrics map to OTel histograms and gauges
- Spans include proper timing via SpanBuilder.with_start_time
- Config: backend="otel", otel_endpoint, otel_service_name
- Accepts "otel", "opentelemetry", "otlp" as backend aliases
- Graceful fallback to NoopObserver on init failure

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

* fix: resolve unused variable warning and update Cargo.lock

Prefix unused `resolved_key` with underscore to suppress clippy
warning introduced by upstream changes. Regenerate Cargo.lock
after rebase on main.

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

* fix: address review comments on OTel observer

- Fix metric types: use Gauge for ActiveSessions/QueueDepth (absolute
  readings, not deltas), Counter<u64> for TokensUsed (monotonic)
- Remove duplicate token recording from AgentEnd event handler
  (TokensUsed metric via record_metric is the canonical path)
- Store meter_provider in struct so flush() exports both traces
  and metrics (was silently dropping metrics on shutdown)

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

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: argenis de la rosa <theonlyhennygod@gmail.com>

src/observability/mod.rs

@@ -1,10 +1,12 @@
 pub mod log;
 pub mod multi;
 pub mod noop;
+pub mod otel;
 pub mod traits;
 
 pub use self::log::LogObserver;
 pub use noop::NoopObserver;
+pub use otel::OtelObserver;
 pub use traits::{Observer, ObserverEvent};
 
 use crate::config::ObservabilityConfig;
@@ -13,6 +15,24 @@ 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)
+                }
+            }
+        }
         "none" | "noop" => Box::new(NoopObserver),
         _ => {
             tracing::warn!(
@@ -32,6 +52,7 @@ mod tests {
     fn factory_none_returns_noop() {
         let cfg = ObservabilityConfig {
             backend: "none".into(),
+            ..ObservabilityConfig::default()
         };
         assert_eq!(create_observer(&cfg).name(), "noop");
     }
@@ -40,6 +61,7 @@ mod tests {
     fn factory_noop_returns_noop() {
         let cfg = ObservabilityConfig {
             backend: "noop".into(),
+            ..ObservabilityConfig::default()
         };
         assert_eq!(create_observer(&cfg).name(), "noop");
     }
@@ -48,14 +70,46 @@ mod tests {
     fn factory_log_returns_log() {
         let cfg = ObservabilityConfig {
             backend: "log".into(),
+            ..ObservabilityConfig::default()
         };
         assert_eq!(create_observer(&cfg).name(), "log");
     }
 
+    #[test]
+    fn factory_otel_returns_otel() {
+        let cfg = ObservabilityConfig {
+            backend: "otel".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_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");
+    }
+
     #[test]
     fn factory_unknown_falls_back_to_noop() {
         let cfg = ObservabilityConfig {
-            backend: "prometheus".into(),
+            backend: "xyzzy_unknown".into(),
+            ..ObservabilityConfig::default()
         };
         assert_eq!(create_observer(&cfg).name(), "noop");
     }
@@ -64,6 +118,7 @@ mod tests {
     fn factory_empty_string_falls_back_to_noop() {
         let cfg = ObservabilityConfig {
             backend: String::new(),
+            ..ObservabilityConfig::default()
         };
         assert_eq!(create_observer(&cfg).name(), "noop");
     }
@@ -72,6 +127,7 @@ mod tests {
     fn factory_garbage_falls_back_to_noop() {
         let cfg = ObservabilityConfig {
             backend: "xyzzy_garbage_123".into(),
+            ..ObservabilityConfig::default()
         };
         assert_eq!(create_observer(&cfg).name(), "noop");
     }