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

cli/test/test_config_generator.py

@@ -386,6 +386,33 @@ model_providers:
     access_key: $OPENAI_API_KEY
     default: true
 
+""",
+    },
+    {
+        "id": "valid_tracing_posthog_exporter",
+        "expected_error": None,
+        "plano_config": """
+version: v0.4.0
+
+listeners:
+  - name: llm
+    type: model
+    port: 12000
+
+model_providers:
+  - model: openai/gpt-4o-mini
+    access_key: $OPENAI_API_KEY
+    default: true
+
+tracing:
+  random_sampling: 100
+  exporters:
+    - type: posthog
+      url: https://us.i.posthog.com
+      api_key: $POSTHOG_API_KEY
+      distinct_id_header: x-user-id
+      capture_messages: false
+
 """,
     },
 ]