fix: Calling Discard only adds to txn_discards metric, not txn_aborts.

[danielmai]

This PR is a follow-up to #7339 to update the way dgraph_txn_discards_total and dgraph_txn_aborts_total are tracked. When the client calls Discard, only the dgraph_txn_discards_total metric is incremented, not dgraph_txn_aborts_total. This makes it easy to see:

1. If the client is calling Discard
2. If the server is aborting transactions (e.g., because of transaction conflicts)

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This change is 

[karlmcguire]

Reviewed 2 of 2 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @manishrjain and @vvbalaji-dgraph)