go.mod: Exclude prometheus/client_golang v1.12.1 (#1027)

See also #1012

As suggested in #1012 (comment)

Signed-off-by: Manuel Rüger <manuel@rueg.eu>

go.mod

@@ -4,6 +4,7 @@ require (
 	github.com/beorn7/perks v1.0.1
 	github.com/cespare/xxhash/v2 v2.1.2
 	github.com/golang/protobuf v1.5.2
+	github.com/jpillora/backoff v1.0.0 // indirect
 	github.com/json-iterator/go v1.1.12
 	github.com/prometheus/client_model v0.2.0
 	github.com/prometheus/common v0.33.0
@@ -12,4 +13,6 @@ require (
 	google.golang.org/protobuf v1.28.0
 )
 
+exclude github.com/prometheus/client_golang v1.12.1
+
 go 1.16