MuonTrackMonitorAlgorithm: Explicitly set merge-type, to avoid errors during histogram merging

Walter Lamplrequested to merge
wlampl/athena:MuTrackMonAlgMergeType into 24.0

Avoids messages like

Info in <TH2F::Add>: Histograms have different labels - trying to use TH1::Merge

during the histogram merging.

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