[7.x] bfetch (2) (#53711)

[streamich]

Backports the following commits to 7.x:

• bfetch (2) (bfetch (2) #53711)

---

Dev Docs

Request batching and response streaming functionality of legacy Interpreter plugin has been moved out into a separate bfetch new platform plugin. Now every plugin can create server endpoints and browser wrappers that can batch HTTP requests and stream responses back.

As an example, we will create a batch processing endpoint that receives a number then doubles it
and streams it back. We will also consider the number to be time in milliseconds
and before streaming the number back the server will wait for the specified number of
milliseconds.

To do that, first create server-side batch processing route using addBatchProcessingRoute.

plugins.bfetch.addBatchProcessingRoute<{ num: number }, { num: number }>(
  '/my-plugin/double',
  () => ({
    onBatchItem: async ({ num }) => {
      // Validate inputs.
      if (num < 0) throw new Error('Invalid number');
      // Wait number of specified milliseconds.
      await new Promise(r => setTimeout(r, num));
      // Double the number and send it back.
      return { num: 2 * num };
    },
  })
);

Now on client-side create double function using batchedFunction.
The newly created double function can be called many times and it
will package individual calls into batches and send them to the server.

const double = plugins.bfetch.batchedFunction<{ num: number }, { num: number }>({
  url: '/my-plugin/double',
});

Note: the created double must accept a single object argument ({ num: number } in this case)
and it will return a promise that resolves into an object, too (also { num: number } in this case).

Use the double function.

double({ num: 1 }).then(console.log, console.error); // { num: 2 }
double({ num: 2 }).then(console.log, console.error); // { num: 4 }
double({ num: 3 }).then(console.log, console.error); // { num: 6 }

[kibanamachine]

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