Add thread pool reject counter to metrics-3.1.2 branch

metrics support thread pool reject counter in 4.0, which is under development (master),
we'are using 3.1.2 and want to use this feature, so fork and patch this feature.


Branch: https://github.com/bitus-dt/metrics/tree/3.1.2-branch
Commit: https://github.com/bitus-dt/metrics/commit/d0c2c0d2e8afd691de82a51701dcd4d793a9761a