docs(metrics): Use correct Badger metrics. (#239)

[MichelDiz]

This PR reflects the update to Dgraph metrics made in dgraph-io/dgraph#7507:

This renames the Badger metrics in the /metrics endpoint to remove the
Badger version in the metric names. This allows the metric name to
remain the same across Badger major versions.

Previous Name	New Name
badger_v3_disk_reads_total	badger_disk_reads_total
badger_v3_disk_writes_total	badger_disk_writes_total
badger_v3_read_bytes	badger_read_bytes
badger_v3_written_bytes	badger_written_bytes
badger_v3_lsm_bloom_hits_total	badger_lsm_bloom_hits_total
badger_v3_gets_total	badger_gets_total
badger_v3_puts_total	badger_puts_total
badger_v3_memtable_gets_total	badger_memtable_gets_total
badger_v3_lsm_size_bytes	badger_lsm_size_bytes
badger_v3_vlog_size_bytes	badger_vlog_size_bytes

This doc change needs to be cherry-picked to release/v21.03.

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