feat: add metrics cost to grpc and http header

[poonai]

---

This change is 

[gja]

Small feedback, but looks good

[gja]

rather than making this a new function, you should probably pass options to queryWithTs(). Or if you don't want to do update all the callers.

type validationOpts struct {
  ResponseCallback func() 
}

func _queryWithTs(..., opts *validationOpts) {
...
context := map[string]interface{}
  if opt.ResponseCallback {
    err := opt.ResponseCallback(context, response)
  }

  return ..., context, error
}

func Test...() {
  ...
  queryWithTs(..., &callbackOptions{ResponseCallback: func() { // put cost into context }})
}

[poonai]

I've made it to return resp instead of passing the validation function.

[gja]

Oh sorry, I should have been explicit 🙈.. Let's keep it similar to graphql.

Dgraph-TouchedUids

[martinmr]

Reviewed 1 of 4 files at r1, 1 of 2 files at r2, 1 of 1 files at r4, 1 of 1 files at r5.
Reviewable status: all files reviewed, 4 unresolved discussions (waiting on @balajijinnah, @gja, @manishrjain, @pawanrawal, and @vvbalaji-dgraph)

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dgraph/cmd/alpha/http_test.go, line 199 at r5 (raw file):

}

func queryWithTsForResp(queryText, contentType, debug string, ts uint64) (string,

Add a comment for these functions so it's clearer what the difference is to the other function with a similar name.

[darkn3rd]

Unit Tests for the Metric?

Reviewed 1 of 4 files at r1, 1 of 2 files at r2, 1 of 1 files at r5, 1 of 1 files at r6.
Reviewable status: all files reviewed, 4 unresolved discussions (waiting on @balajijinnah, @gja, @manishrjain, @pawanrawal, and @vvbalaji-dgraph)

[poonai]

@darkn3rd I've added integration test. That should be enough.