Include the total number of touched nodes in the query metrics.

[martinmr]

Total is stored under a key called "_total".

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This change is 

[manishrjain]

Reviewed 2 of 2 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @pawanrawal)

[danielmai]

If there was a predicate named _total, then this total would conflict with that predicate's num uid count.

We should put the total as another key (called num_uids_total) under metrics:

{
  "metrics": {
    "num_uids": {
      "director.film": 7,
      "name": 46
    }
    "num_uids_total": 53
  }
}