Link prometheus client for bazel grpc build (#908)

RFC=[0014](https://github.com/googlecartographer/rfcs/blob/master/text/0014-monitoring.md)

WORKSPACE

@@ -27,3 +27,9 @@ boost_deps()
 load("@com_github_grpc_grpc//bazel:grpc_deps.bzl", "grpc_deps")
 
 grpc_deps()
+
+load("@com_github_jupp0r_prometheus_cpp//:repositories.bzl", "load_prometheus_client_model", "load_civetweb")
+
+load_prometheus_client_model()
+
+load_civetweb()

bazel/repositories.bzl

@@ -217,6 +217,15 @@ def cartographer_repositories():
       ],
   )
 
+  _maybe(native.http_archive,
+      name = "com_github_jupp0r_prometheus_cpp",
+      sha256 = "701789ff0cc3a5ea7ba78d31b2d18265969f1c7d669f83800fffd8f39c4fa1bb",
+      strip_prefix = "prometheus-cpp-743722db96465aa867bf569eb455ad82dab9f819",
+      urls = [
+          "https://github.com/jupp0r/prometheus-cpp/archive/743722db96465aa867bf569eb455ad82dab9f819.tar.gz",
+      ],
+  )
+
   # TODO(rodrigoq): remove these binds once grpc#14140 has been merged, as well
   # as removing `use_external` in cartographer_grpc/BUILD.bazel.
   # https://github.com/grpc/grpc/pull/14140
@@ -229,7 +238,6 @@ def cartographer_repositories():
       actual = "@com_github_grpc_grpc//:grpc++_codegen_proto",
   )
 
-
 def _maybe(repo_rule, name, **kwargs):
   if name not in native.existing_rules():
     repo_rule(name=name, **kwargs)

cartographer_grpc/BUILD.bazel

@@ -68,6 +68,7 @@ cc_library(
         ":cc_protos",
         "//cartographer",
         "@com_github_grpc_grpc//:grpc++",
+        "@com_github_jupp0r_prometheus_cpp//:prometheus_cpp",
         "@com_google_glog//:glog",
         "@com_google_protobuf//:cc_wkt_protos",
     ],