fix: gh-720 Use express path templates for prometheus metrics

[checketts]

Changes from:

http_request_duration_milliseconds_sum{path="/api/admin/features/myFeature/toggle/on",method="POST",status="200"} 51.16884
http_request_duration_milliseconds_count{path="/api/admin/features/myFeature/toggle/on",method="POST",status="200"} 2

to

http_request_duration_milliseconds_sum{path="/api/admin/features/:featureName/toggle/on",method="POST",status="200"} 51.16884
http_request_duration_milliseconds_count{path="/api/admin/features/:featureName/toggle/on",method="POST",status="200"} 2

Note :featureName. I didn't adopt the express-prometheus-middleware technique, because it just tried to regex out IDs and didn't seem to really solve it. The req had what we needed instead. (Just had to read the docs ;) )

[coveralls]

Coverage decreased (-0.0007%) to 96.023% when pulling 57e84b3 on checketts:prometheus-url-parameters into 8d1851c on Unleash:master.

[chriswk]

Seems like a good way to reduce cardinality of our request metrics. 👍