Fix diagnostic data merge logic for MongoDB

[KeisukeYamashita]

PMM-XXXX (optional, if ticket reported)

• MongoDB 8 0.42.1 panics with MongoDB 8 runs into error duplicated metrics registration #962

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[it-percona-cla]

All committers have signed the CLA.

[BupycHuk]

we've already fixed this problem, release of v0.42.1 was an accident, we will release stable v0.43.0 soon

[KeisukeYamashita]

Yeah, I saw this #961, thank you 👍

[BupycHuk]

it's in #961