MTD[mu, nu] is the metric tensor in D dimensions.
Overview, FeynCalcExternal, FCE, FCI, MT, MTE.
MTD[\[Alpha], \[Beta]]g^{\alpha \beta }
Contract[MTD[\[Alpha], \[Beta]] MTD[\[Alpha], \[Beta]]]D
MTD[\[Alpha], \[Beta]] // StandardForm
(*MTD[\[Alpha], \[Beta]]*)FCI[MTD[\[Alpha], \[Beta]]] // StandardForm
(*Pair[LorentzIndex[\[Alpha], D], LorentzIndex[\[Beta], D]]*)FCE[FCI[MTD[\[Mu], \[Nu]]]] // StandardForm
(*MTD[\[Mu], \[Nu]]*)