update BUILD files

vendor/github.com/prometheus/client_golang/prometheus/BUILD

@@ -23,6 +23,7 @@ go_library(
         "value.go",
         "vec.go",
     ],
+    importpath = "github.com/prometheus/client_golang/prometheus",
     visibility = ["//visibility:public"],
     deps = [
         "//vendor/github.com/beorn7/perks/quantile:go_default_library",

vendor/github.com/prometheus/client_golang/prometheus/promhttp/BUILD

@@ -10,6 +10,7 @@ go_library(
         "instrument_client_1_8.go",
         "instrument_server.go",
     ],
+    importpath = "github.com/prometheus/client_golang/prometheus/promhttp",
     visibility = ["//visibility:public"],
     deps = [
         "//vendor/github.com/prometheus/client_golang/prometheus:go_default_library",