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>

Cargo.toml

@@ -80,6 +80,11 @@ tower = { version = "0.5", default-features = false }
 tower-http = { version = "0.6", default-features = false, features = ["limit", "timeout"] }
 http-body-util = "0.1"
 
+# OpenTelemetry — OTLP trace + metrics export
+opentelemetry = { version = "0.31", default-features = false, features = ["trace", "metrics"] }
+opentelemetry_sdk = { version = "0.31", default-features = false, features = ["trace", "metrics"] }
+opentelemetry-otlp = { version = "0.31", default-features = false, features = ["trace", "metrics", "http-proto", "reqwest-blocking-client"] }
+
 [profile.release]
 opt-level = "z"      # Optimize for size
 lto = true          # Link-time optimization