metrics update - remove legacy

AGENTS.md

@@ -300,17 +300,15 @@ end
 
 ### Metrics Collection
 
-The SDK supports legacy and canonical metric surfaces, selected by the
-`WORKER_CANONICAL_METRICS` environment variable. `MetricsCollector.create`
-returns the appropriate collector:
+`MetricsCollector.create` returns a collector that emits the canonical
+(harmonized) metric surface:
 
 ```ruby
 metrics = Conductor::Worker::Telemetry::MetricsCollector.create(backend: :prometheus)
 ```
 
 See [docs/METRICS_AND_INTERCEPTORS.md](docs/METRICS_AND_INTERCEPTORS.md) for
-the full legacy and canonical metrics catalogs, label reference, and migration
-guide.
+the full metrics catalog and label reference.
 
 ### Worker Configuration (3-Tier Hierarchy)
 
@@ -407,11 +405,8 @@ lib/conductor/
 │   │   ├── listeners.rb          # Listener protocol
 │   │   └── listener_registry.rb  # Registration helper
 │   └── telemetry/                # Metrics
-│       ├── metrics_collector.rb  # Factory (WORKER_CANONICAL_METRICS gate)
-│       ├── legacy_metrics_collector.rb  # Legacy metric set
-│       ├── canonical_metrics_collector.rb # Canonical metric set
-│       ├── prometheus_backend.rb # Legacy Prometheus backend
-│       └── canonical_prometheus_backend.rb # Canonical Prometheus backend
+│       ├── metrics_collector.rb  # MetricsCollector class + NullBackend
+│       └── prometheus_backend.rb # PrometheusBackend + MetricsServer
 └── workflow/
     ├── dsl/                      # Workflow DSL
     │   ├── workflow_builder.rb   # Core DSL engine (~1000 lines)

CHANGELOG.md

@@ -9,7 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
-- Canonical metrics mode: opt-in harmonized metric surface via `WORKER_CANONICAL_METRICS=true` -- [details](docs/METRICS_AND_INTERCEPTORS.md#detailed-technical-notes----unreleased)
+- Canonical (harmonized) metrics as the sole metric surface -- [details](docs/METRICS_AND_INTERCEPTORS.md#detailed-technical-notes----unreleased)
 - Bounded `uri` label on `http_api_client_request_seconds`: uses path templates (e.g. `/workflow/{workflowId}`) instead of fully-resolved paths, preventing metric cardinality explosion
 - `WorkflowStatusProbe` in harness: opt-in probe (via `HARNESS_PROBE_RATE_PER_SEC`) that exercises UUID-bearing endpoints to validate template URI metrics
 
@@ -23,12 +23,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   - Control flow blocks: `parallel do`, `decide expr do`, `loop_over items do`
   - Auto-generated task reference names
   - Simplified LLM task methods with hash-to-ChatMessage auto-conversion
-- `MetricsCollector.new(...)` is deprecated; use `MetricsCollector.create(...)` instead. `.new` still works but logs a deprecation warning. The previous implementation is preserved as `LegacyMetricsCollector` and remains the default.
-- Legacy metrics emit unchanged by default; existing dashboards and alerts continue to work without modification
-- HTTP request timing (`http_api_client_request_seconds`) is now zero-overhead in legacy mode: `RestClient` only enters the timing path when a canonical collector is subscribed to `GlobalDispatcher`
+- HTTP request timing (`http_api_client_request_seconds`) is zero-overhead when no collector is active: `RestClient` only enters the timing path when a `MetricsCollector` is subscribed to `GlobalDispatcher`
 - `thread_uncaught_exceptions_total` is no longer incremented for caught exceptions in the polling loop; the metric surface is retained but unwired, matching the Python and JavaScript SDKs
-- Both `CanonicalMetricsCollector` and `LegacyMetricsCollector` now respond to `stop`; `TaskHandler#stop` calls it automatically to unsubscribe from process-wide dispatchers
-- `MetricsCollector.create` accepts `measure_payload_size:` (default `true` for canonical, `false` for legacy) to opt out of `workflow_input_size_bytes` JSON serialization overhead
+- `MetricsCollector` responds to `stop`; `TaskHandler#stop` calls it automatically to unsubscribe from process-wide dispatchers
+- `MetricsCollector.create` accepts `measure_payload_size:` (default `true`) to opt out of `workflow_input_size_bytes` JSON serialization overhead
 
 ### Removed