Add float support.

Author: vkryachko

encoders/firebase-encoders-proto/src/main/java/com/google/firebase/encoders/proto/ProtobufValueEncoderContext.java

@@ -47,6 +47,14 @@ public ValueEncoderContext add(@Nullable String value) throws IOException {
     return this;
   }
 
+  @NonNull
+  @Override
+  public ValueEncoderContext add(float value) throws IOException {
+    checkNotUsed();
+    objEncoderCtx.add(field, value);
+    return this;
+  }
+
   @NonNull
   @Override
   public ValueEncoderContext add(double value) throws IOException {