Metrics: Adds recall to the api

[manrajgrover]

Description

This PR adds recall to the metrics api

---

For repository owners only:

Please remember to apply all applicable tags to your pull request.
Tags: FEATURE, BREAKING, BUG, PERF, DEV, DOC, SECURITY

For more info see: https://github.com/tensorflow/tfjs/blob/master/DEVELOPMENT.md

---

This change is 

[caisq]

Thanks!

Reviewable status: 0 of 1 approvals obtained (waiting on @manrajgrover, @ericdnielsen, and @bileschi)

---

src/metrics_test.ts, line 248 at r1 (raw file):

const x = tensor2d([[0, 0, 1], [0, 0, 1], [1, 0, 0], [0, 1, 0]]);

I think the edge case for recall is the cases in which there are no 1's in x.

[caisq]

@manrajgrover Heads-up as this PR looks great except the minor testing issue I pointed out in the last comment, I may merge it first and improve the test myself. We are planning a new release soon and I want to include the new metrics in that release.

@ericdnielsen @bileschi please chime in if you have any thoughts.

[caisq]

Thanks for the PR, @manrajgrover !!

Reviewable status: 0 of 1 approvals obtained (waiting on @manrajgrover, @ericdnielsen, and @bileschi)