Scrape admin requests

[eminugurkenar]

this pr adds option to scrape Admin Requests https://github.com/facebook/mcrouter/wiki/admin-requests

those metrics are helpful from operational side, like we can get alerted if one mcrouter config is different than others etc...

example metrics

# HELP mcrouter_admin_request_config_age How long, in seconds, since last config reload.
# TYPE mcrouter_admin_request_config_age gauge
mcrouter_admin_request_config_age 1.043777e+06
# HELP mcrouter_admin_request_config_file Config file location (error if configured from string).
# TYPE mcrouter_admin_request_config_file gauge
mcrouter_admin_request_config_file{file="/opt/mcrouter/config/config.json"} 1
# HELP mcrouter_admin_request_config_md5_digest Current config's md5 hash. Note that this only specifies the main config file's md5 and ignores any additional tracked files.
# TYPE mcrouter_admin_request_config_md5_digest gauge
mcrouter_admin_request_config_md5_digest{hash="2aa22ce671e9fdf6a7bb762f9a6cb0cc"} 1
# HELP mcrouter_admin_request_host_id Hostid of this mcrouter instance, an unsigned 32-bit integer in decimal.
# TYPE mcrouter_admin_request_host_id gauge
mcrouter_admin_request_host_id{host_id="1938114997"} 1
# HELP mcrouter_admin_request_version Version string of the build (same string as returned by version).
# TYPE mcrouter_admin_request_version gauge
mcrouter_admin_request_version{version="40.0.0 mcrouter"} 1
# HELP mcrouter_asynclog_requests Number of failed deletes written to spool file.
# TYPE mcrouter_asynclog_requests counter