[perf][android] Add documentation

android/src/main/java/io/invertase/firebase/perf/RNFirebasePerformance.java

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+package io.invertase.firebase.perf;
+
+
+import android.util.Log;
+
+import com.facebook.react.bridge.ReactApplicationContext;
+import com.facebook.react.bridge.ReactContextBaseJavaModule;
+import com.facebook.react.bridge.ReactMethod;
+import com.google.firebase.perf.FirebasePerformance;
+import com.google.firebase.perf.metrics.Trace;
+
+import java.util.HashMap;
+
+public class RNFirebasePerformance extends ReactContextBaseJavaModule {
+
+  private static final String TAG = "RNFirebasePerformance";
+  private HashMap<String, Trace> traces = new HashMap<>();
+
+  public RNFirebasePerformance(ReactApplicationContext reactContext) {
+    super(reactContext);
+    Log.d(TAG, "New instance");
+  }
+
+  /**
+   * @return
+   */
+  @Override
+  public String getName() {
+    return TAG;
+  }
+
+  @ReactMethod
+  public void start(String identifier) {
+    Trace trace = getOrCreateTrace(identifier);
+    trace.start();
+  }
+
+  @ReactMethod
+  public void stop(String identifier) {
+    Trace trace = getOrCreateTrace(identifier);
+    trace.stop();
+  }
+
+  @ReactMethod
+  public void incrementCounter(String identifier, String event) {
+    Trace trace = getOrCreateTrace(identifier);
+    trace.incrementCounter(event);
+  }
+
+  private Trace getOrCreateTrace(String identifier) {
+    if (traces.containsKey(identifier)) {
+      return traces.get(identifier);
+    }
+    Trace trace = FirebasePerformance.getInstance().newTrace(identifier);
+    traces.put(identifier, trace);
+    return trace;
+  }
+}

docs/modules/perf.md

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+# Performance Monitoring
+
+!> Performance monitoring requires react-native-firebase version 1.2.0.
+
+?> If you plan on using this module in your own application, please ensure the optional setup instructions for
+[Android](#) and [iOS](#) have been followed.
+
+Out of the box, [Firebase Performance Monitoring](https://firebase.google.com/docs/perf-mon/automatic) monitors a number of
+[automatic traces](https://firebase.google.com/docs/perf-mon/automatic) such as app start/background/foreground response times.
+You can easily trace your own events with RNFirebase:
+
+## API
+
+#### newTrace(id: `string`): `Trace`
+
+Returns a new instance of Trace (see API below). The id is the unique name of something you'd like to run performance
+monitoring against.
+
+```js
+const trace = firebase.perf().newTrace("test_trace");
+```
+
+### Trace
+
+#### start()
+
+Initializes the trace to start tracing performance to relay back to Firebase.
+
+```js
+trace.start();
+```
+
+#### incrementCounter(event: string)
+
+Notifies Firebase an event has occured. These events will be visible on Firebase once your trace has stopped.
+
+```js
+someCacheService.get('user:123')
+  .then((user) => {
+    if (user) {
+      trace.incrementCounter('user_cache_hit');
+    } else {
+      trace.incrementCounter('user_cache_missed');
+    }
+  });
+```
+
+#### stop()
+
+Stops performance tracing. The completed trace stats are now sent to Firebase.
+
+```js
+trace.stop();
+```