Speed up shutdown according to remaining active connections

Changelog for 0.9.0:

• PR #125 Speed up shutdown according to number of connections by @alexellis

Commits
aeccace Add metrics to README by @alexellis
a265b19 Upgrade Prometheus client library to v1.11.0 by @alexellis
31e1d32 Perform graceful shutdown according to number of connections by @alexellis
989ac5f Update README.md by @alexellis

Changes: 0.8.4...0.9.0

Generated by Derek