Java: Add OpenFeign integration docs.

Fixes getsentry#3971

src/platforms/java/performance/instrumentation/open-feign.mdx

@@ -0,0 +1,96 @@
+---
+title: OpenFeign Integration
+sidebar_order: 20
+description: "Learn how to capture OpenFeign based HTTP clients performance"
+---
+
+<Note>
+
+Capturing transactions requires that you first <PlatformLink to="/performance/">set up performance monitoring</PlatformLink> if you haven't already.
+
+</Note>
+
+Sentry OpenFeign integration provides `SentryFeignClient` that creates a span for each outgoing HTTP request executed with a [Feign](https://github.com/OpenFeign/feign) based HTTP client.
+
+### Install
+
+```xml {tabTitle:Maven}
+<dependency>
+    <groupId>io.sentry</groupId>
+    <artifactId>sentry-openfeign</artifactId>
+    <version>{{ packages.version('sentry.java.openfeign', '5.1.0') }}</version>
+</dependency>
+```
+
+```groovy {tabTitle:Gradle}
+implementation 'io.sentry:sentry-openfeign:{{ packages.version('sentry.java.openfeign', '5.1.0') }}'
+```
+
+```scala {tabTitle: SBT}
+libraryDependencies += "io.sentry" % "sentry-openfeign" % "{{ packages.version('sentry.java.openfeign', '5.1.0') }}"
+```
+
+For other dependency managers see the [central Maven repository](https://search.maven.org/artifact/io.sentry/sentry-openfeign).
+
+## Configure
+
+Add `SentryCapability` to Feign builder:
+
+```java
+import feign.Feign;
+import io.sentry.openfeign.SentryCapability;
+
+YourApi api = Feign.builder()
+    .addCapability(new SentryCapability())
+    ...
+    .target(YourApi.class, "https://your-api-host/");
+```
+
+```kotlin
+import feign.Feign
+import io.sentry.openfeign.SentryCapability
+
+val api = Feign.builder()
+    .addCapability(SentryCapability())
+    ...
+    .target(YourApi::class.java, "https://your-api-host/")
+```
+
+## Modify or Drop spans
+
+Spans created around HTTP requests can be modified or dropped using `SentryFeignClient.BeforeSpanCallback` passed to `SentryCapability`:
+
+```java
+import feign.Feign;
+import io.sentry.openfeign.SentryCapability;
+
+YourApi api = Feign.builder()
+    .addCapability(
+        new SentryCapability(
+            (span, request, response) -> {
+              // modify span or return `null` to drop
+              if (request.url().endsWith("/todos")) {
+                span.setTag("tag-name", "tag-value");
+              }
+              return span;
+            }))
+    ...
+    .target(YourApi.class, "https://your-api-host/");
+```
+
+```kotlin
+import feign.Feign
+import io.sentry.openfeign.SentryCapability
+
+val api = Feign.builder()
+    .addCapability(
+        SentryCapability(
+            BeforeSpanCallback { span: ISpan, request: Request, response: Response? ->
+                // modify span or return `null` to drop
+                if (request.url().endsWith("/todos")) {
+                    span.setTag("tag-name", "tag-value")
+                }
+                span
+            }))
+    .target(TodoApi::class.java, "https://jsonplaceholder.typicode.com/")
+```