bump go-grpc-prometheus since 1.2.0 actually exposes the metrics

v1.1.0 (which is what we currently use), does not expose metrics which
makes it impossible to migrate.

vendor/github.com/grpc-ecosystem/go-grpc-prometheus/makefile

@@ -0,0 +1,16 @@
+SHELL="/bin/bash"
+
+GOFILES_NOVENDOR = $(shell go list ./... | grep -v /vendor/)
+
+all: vet fmt test
+
+fmt:
+	go fmt $(GOFILES_NOVENDOR)
+
+vet:
+	go vet $(GOFILES_NOVENDOR)
+
+test: vet
+	./scripts/test_all.sh
+
+.PHONY: all vet test