Add README for dd-openfeature with eval metrics setup

Documents the published artifact setup, evaluation metrics
dependencies (opentelemetry-sdk-metrics, opentelemetry-exporter-otlp),
OTLP endpoint configuration, metric attributes, and requirements.

Author: typotter

products/feature-flagging/feature-flagging-api/README.md

@@ -0,0 +1,82 @@
+# dd-openfeature
+
+Datadog OpenFeature Provider for Java. Implements the [OpenFeature](https://openfeature.dev/) `FeatureProvider` interface for Datadog's Feature Flags and Experimentation (FFE) product.
+
+Published as `com.datadoghq:dd-openfeature` on Maven Central.
+
+## Setup
+
+```xml
+<dependency>
+    <groupId>com.datadoghq</groupId>
+    <artifactId>dd-openfeature</artifactId>
+    <version>${dd-openfeature.version}</version>
+</dependency>
+<dependency>
+    <groupId>dev.openfeature</groupId>
+    <artifactId>sdk</artifactId>
+    <version>1.20.1</version>
+</dependency>
+```
+
+### Evaluation metrics (optional)
+
+To enable evaluation metrics (`feature_flag.evaluations` counter), add the OpenTelemetry SDK dependencies:
+
+```xml
+<dependency>
+    <groupId>io.opentelemetry</groupId>
+    <artifactId>opentelemetry-sdk-metrics</artifactId>
+    <version>1.47.0</version>
+</dependency>
+<dependency>
+    <groupId>io.opentelemetry</groupId>
+    <artifactId>opentelemetry-exporter-otlp</artifactId>
+    <version>1.47.0</version>
+</dependency>
+```
+
+Any OpenTelemetry API 1.x version is compatible. If these dependencies are absent, the provider operates normally without metrics.
+
+## Usage
+
+```java
+import datadog.trace.api.openfeature.Provider;
+import dev.openfeature.sdk.OpenFeatureAPI;
+import dev.openfeature.sdk.Client;
+
+OpenFeatureAPI api = OpenFeatureAPI.getInstance();
+api.setProviderAndWait(new Provider());
+Client client = api.getClient();
+
+boolean enabled = client.getBooleanValue("my-feature", false,
+    new MutableContext("user-123"));
+```
+
+## Evaluation metrics
+
+When the OTel SDK dependencies are on the classpath, the provider records a `feature_flag.evaluations` counter via OTLP HTTP/protobuf. Metrics are exported every 10 seconds to the Datadog Agent's OTLP receiver.
+
+### Configuration
+
+| Environment variable | Description | Default |
+|---|---|---|
+| `OTEL_EXPORTER_OTLP_METRICS_ENDPOINT` | Signal-specific OTLP endpoint (used as-is) | — |
+| `OTEL_EXPORTER_OTLP_ENDPOINT` | Generic OTLP endpoint (`/v1/metrics` appended) | — |
+| (none set) | Default endpoint | `http://localhost:4318/v1/metrics` |
+
+### Metric attributes
+
+| Attribute | Description |
+|---|---|
+| `feature_flag.key` | Flag key |
+| `feature_flag.result.variant` | Resolved variant key |
+| `feature_flag.result.reason` | Evaluation reason (lowercased) |
+| `error.type` | Error code (lowercased, only on error) |
+| `feature_flag.result.allocation_key` | Allocation key (when present) |
+
+## Requirements
+
+- Java 11+
+- Datadog Agent with Remote Configuration enabled
+- `DD_EXPERIMENTAL_FLAGGING_PROVIDER_ENABLED=true`