[build] Add missing checksum (grpc#37613)

Just remove a warning I saw doing a clean build.

Closes grpc#37613

COPYBARA_INTEGRATE_REVIEW=grpc#37613 from eugeneo:add-sha e654642
PiperOrigin-RevId: 670602715

bazel/grpc_deps.bzl

@@ -304,6 +304,7 @@ def grpc_deps():
     if "com_github_jupp0r_prometheus_cpp" not in native.existing_rules():
         http_archive(
             name = "com_github_jupp0r_prometheus_cpp",
+            sha256 = "cdf03ee63fcb1d9e113f7bd525e043c254729dddf19d80396489f5b92c83c18d",
             strip_prefix = "prometheus-cpp-b1234816facfdda29845c46696a02998a4af115a",
             urls = [
                 "https://github.com/jupp0r/prometheus-cpp/archive/b123481.zip",