Send `X-Metro-Delta-ID` header

Summary:
Adds additional headers to delta responses:
- The next delta ID. This helps to defer parsing the delta payload to a different layer.
- The size of the delta. This can be used as a hint for memory allocation

Reviewed By: jeanlauliac

Differential Revision: D7428603

fbshipit-source-id: 1d5e840991ec8cb490d33b3bc97444b0d7a3a0f3

packages/metro/src/Server/__tests__/Server-test.js

@@ -433,6 +433,8 @@ describe('processRequest', () => {
         ],
         reset: true,
       });
+
+      expect(response.headers['X-Metro-Delta-ID']).toEqual('XXXXX-0');
     });
 
     it('should generate an incremental delta correctly', async () => {
@@ -469,6 +471,8 @@ describe('processRequest', () => {
         reset: false,
       });
 
+      expect(response.headers['X-Metro-Delta-ID']).toEqual('XXXXX-1');
+
       expect(DeltaBundler.prototype.getDelta.mock.calls[0][1]).toEqual({
         reset: false,
       });

packages/metro/src/Server/index.js

@@ -84,6 +84,7 @@ function debounceAndBatch(fn, delay) {
   };
 }
 
+const DELTA_ID_HEADER = 'X-Metro-Delta-ID';
 const FILES_CHANGED_COUNT_HEADER = 'X-Metro-Files-Changed-Count';
 
 class Server {
@@ -687,12 +688,11 @@ class Server {
       }),
     );
 
-    let output;
+    let output, sequenceId;
 
     try {
-      const {delta, graph, prepend, sequenceId} = await this._getDeltaInfo(
-        options,
-      );
+      let delta, graph, prepend;
+      ({delta, graph, prepend, sequenceId} = await this._getDeltaInfo(options));
 
       output = {
         bundle: deltaJSBundle(
@@ -724,7 +724,8 @@ class Server {
     }
 
     mres.setHeader(FILES_CHANGED_COUNT_HEADER, String(output.numModifiedFiles));
-    mres.setHeader('Content-Type', 'application/javascript');
+    mres.setHeader(DELTA_ID_HEADER, String(sequenceId));
+    mres.setHeader('Content-Type', 'application/json');
     mres.setHeader('Content-Length', String(Buffer.byteLength(output.bundle)));
     mres.end(output.bundle);