feat(java): Add automatic and custom performance metrics doc

src/platform-includes/performance/automatic-performance-metrics/android.mdx

@@ -0,0 +1,4 @@
+If configured, the Android SDK automatically collects the following performance metrics:
+
+* Cold and warm app start time - learn more in [App Start Instrumentation](/platforms/android/performance/instrumentation/automatic-instrumentation/#app-start-instrumentation).
+* Frozen and slow frame rendering - learn more in [Slow and Frozen Frames](/platforms/android/performance/instrumentation/automatic-instrumentation/#slow-and-frozen-frames).

src/platform-includes/performance/custom-performance-metrics/java.mdx

@@ -0,0 +1,36 @@
+Adding custom metrics is supported in Sentry's Java SDK version `6.5.0` and above.
+
+```java {tabTitle:Java}
+import io.sentry.ISpan;
+import io.sentry.MeasurementUnit;
+import io.sentry.Sentry;
+
+final ISpan span = Sentry.getSpan();
+if (span != null) {
+    // Record amount of memory used
+    span.setMeasurement("memory_used", 64, MeasurementUnit.Information.MEGABYTE);
+
+    // Record time it took to load user profile
+    span.setMeasurement("user_profile_loading_time", 1.3, MeasurementUnit.Duration.SECOND);
+
+    // Record number of times the screen was loaded
+    span.setMeasurement("screen_load_count", 4);
+}
+```
+
+
+```kotlin {tabTitle:Kotlin}
+import io.sentry.MeasurementUnit
+import io.sentry.Sentry
+
+val span = Sentry.getSpan()
+
+// Record amount of memory used
+span?.setMeasurement("memory_used", 64, MeasurementUnit.Information.MEGABYTE)
+
+// Record time it took to load user profile
+span?.setMeasurement("user_profile_loading_time", 1.3, MeasurementUnit.Duration.SECOND)
+
+// Record number of times the screen was loaded
+span?.setMeasurement("screen_load_count", 4)
+```

src/platforms/common/performance/instrumentation/performance-metrics.mdx

@@ -7,15 +7,15 @@ supported:
   - dart
   - flutter
   - react-native
+  - java
+  - java.spring-boot
+  - android
 notSupported:
   - javascript.cordova
   - javascript.electron
   - dotnet
   - go
-  - java
-  - android
   - ruby
-  - java.spring-boot
   - apple
   - unity
   - rust
@@ -31,7 +31,7 @@ description: "Learn how to attach performance metrics to your transactions."
 
 Sentry's SDKs support sending performance metrics data to Sentry. These are numeric values attached to transactions that are aggregated and displayed in Sentry.
 
-<PlatformSection supported={["javascript"]} notSupported={["react-native"]}>
+<PlatformSection supported={["javascript", "android"]} notSupported={["react-native"]}>
 
 <PlatformContent includePath="performance/automatic-performance-metrics" />