feat(metrics): Add Dgraph txn metrics (commits and discards).

[danielmai]

In #6171 the metric to track txn aborts was added. This PR adds metrics for txn commits and txn discards:

• dgraph_txn_commits_total
• dgraph_txn_discards_total

dgraph_txn_commits_total is incremented on successful commits.

dgraph_txn_discards_total is incremented when the client calls txn.Discard. Calling txn.Discard is also considered a txn abort, so dgraph_txn_aborts_total is also incremented (that's already the behavior even before this change). As of #7365 dgraph_txn_discards_total tracks the number of client-called Discard calls. dgraph_txn_aborts_total tracks the number of txn aborts made by the server.

Changes to dgraph_txn metrics (including the existing dgraph_txn_aborts_total metric):

When the method tag is exported it's possible to get split metrics like
the following:

dgraph_txn_aborts_total{method="",status=""} 2
dgraph_txn_aborts_total{method="Server.Mutate",status=""} 1
dgraph_txn_commits_total{method="",status=""} 3
dgraph_txn_commits_total{method="Server.Mutate",status=""} 13
dgraph_txn_discards_total{method="",status=""} 1

When setting "CommitNow" in a mutation, the txn commit and abort metrics
are tracked under method="Server.Mutate". When the mutate and commit are
separate calls, the metric is recorded under method="".

This is confusing. It's clearer to bucket these metrics together, so for
the dgraph_txn metrics we don't set those tags in the stats view:

dgraph_txn_aborts_total 3
dgraph_txn_commits_total 16
dgraph_txn_discards_total 1

Fixes DGRAPH-1137

---

This change is 

[netlify]

Deploy preview for dgraph-docs ready!

Built with commit 71222ad

https://deploy-preview-7339--dgraph-docs.netlify.app

[manishrjain]

Reviewed 1 of 3 files at r8, 3 of 3 files at r10, 1 of 1 files at r11.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @danielmai and @vvbalaji-dgraph)

---

worker/draft.go, line 1770 at r11 (raw file):

func (n *node) monitorRaftMetrics() {
	ticker := time.NewTicker(5 * time.Second)

revert.

---

x/metrics.go, line 293 at r11 (raw file):

			TagKeys:     allTagKeys,
		},
    // Raft metrics

the comment is not aligned.