Add flag evaluation error message

docs/attributes-registry/feature-flag.md

@@ -17,6 +17,7 @@ This document defines attributes for Feature Flags.
 |---|---|---|---|---|
 | <a id="feature-flag-context-id" href="#feature-flag-context-id">`feature_flag.context.id`</a> | string | The unique identifier for the flag evaluation context. For example, the targeting key. | `5157782b-2203-4c80-a857-dbbd5e7761db` | ![Experimental](https://img.shields.io/badge/-experimental-blue) |
 | <a id="feature-flag-error-code" href="#feature-flag-error-code">`feature_flag.error.code`</a> | string | An error code which shows why a flag evaluation failed | `provider_not_ready`; `targeting_key_missing`; `provider_fatal`; `general` | ![Experimental](https://img.shields.io/badge/-experimental-blue) |
+| <a id="feature-flag-error-message" href="#feature-flag-error-message">`feature_flag.error.message`</a> | string | A message explaining why an error occurred during flag evaluation. | `Flag `header-color` expected type `string` but found type `number`` | ![Experimental](https://img.shields.io/badge/-experimental-blue) |
 | <a id="feature-flag-key" href="#feature-flag-key">`feature_flag.key`</a> | string | The lookup key of the feature flag. | `logo-color` | ![Experimental](https://img.shields.io/badge/-experimental-blue) |
 | <a id="feature-flag-reason" href="#feature-flag-reason">`feature_flag.reason`</a> | string | The reason code which shows how a feature flag value was determined. | `static`; `targeting_match`; `error`; `default` | ![Experimental](https://img.shields.io/badge/-experimental-blue) |
 | <a id="feature-flag-set-id" href="#feature-flag-set-id">`feature_flag.set.id`</a> | string | The identifier of the [flag set](https://openfeature.dev/specification/glossary/#flag-set) which the feature flag belongs to in a flag management system. | `proj-1`; `ab98sgs`; `service1/dev` | ![Experimental](https://img.shields.io/badge/-experimental-blue) |

docs/feature-flags/feature-flags-logs.md

@@ -59,20 +59,25 @@ This semantic convention defines the attributes used to represent a feature flag
 | Attribute  | Type | Description  | Examples  | [Requirement Level](https://opentelemetry.io/docs/specs/semconv/general/attribute-requirement-level/) | Stability |
 |---|---|---|---|---|---|
 | [`feature_flag.key`](/docs/attributes-registry/feature-flag.md) | string | The lookup key of the feature flag. | `logo-color` | `Required` | ![Experimental](https://img.shields.io/badge/-experimental-blue) |
-| [`feature_flag.error.code`](/docs/attributes-registry/feature-flag.md) | string | An error code which shows why a flag evaluation failed | `provider_not_ready`; `targeting_key_missing`; `provider_fatal`; `general` | `Conditionally Required` If an error occurred during flag evaluation. | ![Experimental](https://img.shields.io/badge/-experimental-blue) |
-| [`feature_flag.variant`](/docs/attributes-registry/feature-flag.md) | string | A semantic identifier for an evaluated flag value. [1] | `red`; `true`; `on` | `Conditionally Required` [2] | ![Experimental](https://img.shields.io/badge/-experimental-blue) |
+| [`feature_flag.error.code`](/docs/attributes-registry/feature-flag.md) | string | An error code which shows why a flag evaluation failed | `provider_not_ready`; `targeting_key_missing`; `provider_fatal`; `general` | `Conditionally Required` [1] | ![Experimental](https://img.shields.io/badge/-experimental-blue) |
+| [`feature_flag.error.message`](/docs/attributes-registry/feature-flag.md) | string | A message explaining why an error occurred during flag evaluation. | `Flag `header-color` expected type `string` but found type `number`` | `Conditionally Required` [2] | ![Experimental](https://img.shields.io/badge/-experimental-blue) |
+| [`feature_flag.variant`](/docs/attributes-registry/feature-flag.md) | string | A semantic identifier for an evaluated flag value. [3] | `red`; `true`; `on` | `Conditionally Required` [4] | ![Experimental](https://img.shields.io/badge/-experimental-blue) |
 | [`feature_flag.context.id`](/docs/attributes-registry/feature-flag.md) | string | The unique identifier for the flag evaluation context. For example, the targeting key. | `5157782b-2203-4c80-a857-dbbd5e7761db` | `Recommended` | ![Experimental](https://img.shields.io/badge/-experimental-blue) |
 | [`feature_flag.reason`](/docs/attributes-registry/feature-flag.md) | string | The reason code which shows how a feature flag value was determined. | `static`; `targeting_match`; `error`; `default` | `Recommended` | ![Experimental](https://img.shields.io/badge/-experimental-blue) |
 | [`feature_flag.set.id`](/docs/attributes-registry/feature-flag.md) | string | The identifier of the [flag set](https://openfeature.dev/specification/glossary/#flag-set) which the feature flag belongs to in a flag management system. | `proj-1`; `ab98sgs`; `service1/dev` | `Recommended` | ![Experimental](https://img.shields.io/badge/-experimental-blue) |
 | [`feature_flag.system`](/docs/attributes-registry/feature-flag.md) | string | Identifies the feature flag provider. | `Flag Manager` | `Recommended` | ![Experimental](https://img.shields.io/badge/-experimental-blue) |
 | [`feature_flag.version`](/docs/attributes-registry/feature-flag.md) | string | The version of the ruleset used during the evaluation. This may be any stable value which uniquely identifies the ruleset. | `1`; `01ABCDEF` | `Recommended` | ![Experimental](https://img.shields.io/badge/-experimental-blue) |
 
-**[1]:** A semantic identifier, commonly referred to as a variant, provides a means
+**[1]:** Required if an error occured during flag evaluation and `feature_flag.error.message` is not set, recommended otherwise.
+
+**[2]:** Required if an error occured during flag evaluation and `feature_flag.error.code` is not set, recommended otherwise.
+
+**[3]:** A semantic identifier, commonly referred to as a variant, provides a means
 for referring to a value without including the value itself. This can
 provide additional context for understanding the meaning behind a value.
 For example, the variant `red` maybe be used for the value `#c05543`.
 
-**[2]:** If feature flag provider supplies a variant or equivalent concept.
+**[4]:** If feature flag provider supplies a variant or equivalent concept.
 
 `feature_flag.error.code` has the following list of well-known values. If one of them applies, then the respective value MUST be used; otherwise, a custom value MAY be used.
 

model/feature-flag/logs.yaml

@@ -29,7 +29,10 @@ groups:
         requirement_level: recommended
       - ref: feature_flag.error.code
         requirement_level:
-          conditionally_required: If an error occurred during flag evaluation.
+          conditionally_required: Required if an error occured during flag evaluation and `feature_flag.error.message` is not set, recommended otherwise.
+      - ref: feature_flag.error.message
+        requirement_level:
+          conditionally_required: Required if an error occured during flag evaluation and `feature_flag.error.code` is not set, recommended otherwise.
     body:
       id: feature_flag.evaluation
       type: map

model/feature-flag/registry.yaml

@@ -127,3 +127,8 @@ groups:
         examples: ["provider_not_ready", "targeting_key_missing", "provider_fatal", "general"]
         brief: >
           An error code which shows why a flag evaluation failed
+      - id: feature_flag.error.message
+        type: string
+        stability: experimental
+        examples: ["Flag `header-color` expected type `string` but found type `number`"]
+        brief: A message explaining why an error occurred during flag evaluation.