golang/oauth2/stsexchange: pass partial client authentification info

[olefirenque]

Hi!
I chose oauth2/google/externalaccount package as the basis for my implementation of token-exchange grant-type on the client side.
As I see it, this token exchange configuration supports providing a ClientID:

oauth2/google/externalaccount/basecredentials.go

Line 168 in 3e64809

ClientID string

But it is only used in conjunction with ClientSecret, which is not required for my use, because the SubjectToken itself is used to authenticate the user:

oauth2/google/internal/stsexchange/clientauth.go

Line 27 in 3e64809

if c.ClientID == "" || c.ClientSecret == "" || values == nil || headers == nil {

In my use case, the verified ClientID is extracted from the SubjectToken, but I would like to provide the unverified ClientID as a parameter to use for ratelimiting/metric export purposes, etc.

Is it possible to weaken this check to allow only the ClientID to be specified? Or is there a better way to provide this information?