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