Add the ability to trace methods in internal builds.

Currently only for internal builds. Use the @Trace annotation to trace
methods for viewing in Perfetto.

app/src/main/java/org/thoughtcrime/securesms/tracing/Trace.java

@@ -0,0 +1,14 @@
+package org.thoughtcrime.securesms.tracing;
+
+import java.lang.annotation.Retention;
+import java.lang.annotation.Target;
+
+import static java.lang.annotation.ElementType.CONSTRUCTOR;
+import static java.lang.annotation.ElementType.METHOD;
+import static java.lang.annotation.ElementType.TYPE;
+import static java.lang.annotation.RetentionPolicy.CLASS;
+
+@Target({TYPE, METHOD, CONSTRUCTOR}) @Retention(CLASS)
+public @interface Trace {
+}
+

app/src/main/java/org/thoughtcrime/securesms/tracing/Tracer.java

@@ -0,0 +1,45 @@
+package org.thoughtcrime.securesms.tracing;
+
+import androidx.annotation.NonNull;
+
+/**
+ * A class to create Perfetto-compatible traces.
+ */
+public interface Tracer {
+
+  TracerImpl INSTANCE = new TracerImpl();
+
+  static @NonNull Tracer getInstance() {
+    return INSTANCE;
+  }
+
+  /**
+   * True if enabled, otherwise false.
+   */
+  boolean isEnabled();
+
+  /**
+   * Marks the start of a method call. Always follow this with a call to {@link #end(String)}.
+   */
+  void start(@NonNull String methodName);
+
+  /**
+   * Marks the start of a method call. Always follow this with a call to {@link #end(String)}.
+   *
+   * Includes the ability to pass a key-value pair that will be shown in the trace when you click
+   * on the slice.
+   */
+  void start(@NonNull String methodName, @NonNull String key, @NonNull String value);
+
+  /**
+   * Marks the end of a method call.
+   */
+  void end(@NonNull String methodName);
+
+  /**
+   * Serializes the current state of the trace to a Perfetto-compatible byte array. Note that
+   * there's no locking here, and therefore tracing will continue. We're just grabbing a best-effort
+   * snapshot.
+   */
+  @NonNull byte[] serialize();
+}