Typo in metrics names

[jens-solarisbank]

Nomad version

Current master branch

Issue

Several metric names have typos:

check_regisrations
check_deregisrations
service_regisrations
service_deregisrations

Those should probably be named _regis_t_rations

Reproduction steps

curl localhost:4646/v1/metrics?format=prometheus |grep regisrations

[chelseakomlo]

Closing due to merge of #3611

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