MTE[mu, nu] is the metric tensor in D-4 dimensions.
Overview, FeynCalcExternal, FCE, FCI, MT, MTD.
MTE[\[Alpha], \[Beta]]\hat{g}^{\alpha \beta }
Contract[MTE[\[Alpha], \[Beta]] MTE[\[Alpha], \[Beta]]]D-4
Contract[MTE[\[Alpha], \[Beta]] MT[\[Alpha], \[Beta]]]0
Contract[MTE[\[Alpha], \[Beta]] MTD[\[Alpha], \[Beta]]]D-4
MTE[\[Alpha], \[Beta]] // StandardForm
(*MTE[\[Alpha], \[Beta]]*)MTE[\[Alpha], \[Beta]]\hat{g}^{\alpha \beta }
FCI[MTE[\[Alpha], \[Beta]]] // StandardForm
(*Pair[LorentzIndex[\[Alpha], -4 + D], LorentzIndex[\[Beta], -4 + D]]*)FCE[FCI[MTE[\[Mu], \[Nu]]]] // StandardForm
(*MTE[\[Mu], \[Nu]]*)MTE[\[Mu], \[Nu]]\hat{g}^{\mu \nu }