feat(tracing): provider-agnostic exporters with first-class PostHog support (#972)

* feat(tracing): add provider-agnostic exporters with first-class PostHog support

* chore(config): regenerate full reference rendered config for exporters

* refactor(tracing): drop posthog exporter 'enabled' flag per review

docs/source/resources/includes/plano_config_full_reference.yaml

@@ -261,3 +261,16 @@ tracing:
     static:
       environment: production
       service.team: platform
+  # Provider-agnostic export destinations. LLM spans are streamed to each of
+  # these in addition to any opentracing_grpc_endpoint above.
+  exporters:
+    # PostHog AI observability: each LLM call is captured as an $ai_generation event.
+    - type: posthog
+      # PostHog host. The /batch/ capture path is appended automatically.
+      url: https://us.i.posthog.com
+      # PostHog project API key (token). Supports $ENV_VAR expansion.
+      api_key: $POSTHOG_API_KEY
+      # Optional: request header used as the PostHog distinct_id. Omit for anonymous capture.
+      distinct_id_header: x-user-id
+      # Optional: include the (truncated) user message as $ai_input. Defaults to false.
+      capture_messages: false

docs/source/resources/includes/plano_config_full_reference_rendered.yaml

@@ -266,6 +266,12 @@ system_prompt: 'You are a helpful assistant. Always respond concisely and accura
 
   '
 tracing:
+  exporters:
+  - api_key: $POSTHOG_API_KEY
+    capture_messages: false
+    distinct_id_header: x-user-id
+    type: posthog
+    url: https://us.i.posthog.com
   opentracing_grpc_endpoint: http://localhost:4317
   random_sampling: 100
   span_attributes: