PMM-9027 Fix MongoDB replication lag graph aggregation

[artazar]

The previous aggregation showed same max replication lag value for all replicas in the set, taking the largest value across all replicas. With switching the aggregation filter from service_name to name, the graph should correctly show the lag value for each replica set member.

[it-percona-cla]

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[artazar]

@adivinho thank you, fixed the legendFormat too.