docs(metrics): mark GuardianCheck deprecated and document Intrinsics telemetry gap

Guardian Intrinsics are not Requirement subclasses and emit no
mellea.requirement.checks/failures metrics. Users migrating from
GuardianCheck would otherwise lose those counters silently.

Also fix "Determine is" → "Determine if" typo in factuality_detection
docstring.

Assisted-by: Claude Code

Author: planetf1

docs/docs/observability/metrics.md

@@ -246,9 +246,17 @@ All sampling metrics include:
 
 | Attribute | Description | Example Values |
 | --------- | ----------- | -------------- |
-| `requirement` | Requirement class name | `LLMaJRequirement`, `PythonExecutionReq`, `ALoraRequirement`, `GuardianCheck` |
+| `requirement` | Requirement class name | `LLMaJRequirement`, `PythonExecutionReq`, `ALoraRequirement`, `GuardianCheck` *(deprecated v0.4)* |
 | `reason` | Human-readable failure reason (`mellea.requirement.failures` only) | `"Output did not satisfy constraint"`, `"unknown"` |
 
+> **Guardian Intrinsics and metrics:** `guardian_check()`, `policy_guardrails()`,
+> `factuality_detection()`, and `factuality_correction()` are not `Requirement`
+> subclasses and do not emit `mellea.requirement.checks` or
+> `mellea.requirement.failures` metrics. If you migrate from `GuardianCheck` to
+> Guardian Intrinsics, Guardian-related requirement counters will stop appearing
+> in your metrics. Wrap Guardian Intrinsic calls in a custom `Requirement` subclass
+> if you need to preserve this telemetry.
+
 ### Tool counter
 
 | Metric Name | Type | Unit | Description |

mellea/stdlib/components/intrinsic/guardian.py

@@ -160,7 +160,7 @@ def guardian_check(
 
 
 def factuality_detection(context: ChatContext, backend: AdapterMixin) -> float:
-    """Determine is the last response is factually incorrect.
+    """Determine if the last response is factually incorrect.
 
     Intrinsic function that evaluates the factuality of the
     assistant's response to a user's question. The context should end with