Feature/http call duration metrics

[SimoneBergonzi]

Add metrics to collect http client request duration.
The metric is enabled at service level by mean of the variable ENABLE_HTTP_CLIENT_METRICS (default: false).
The metric can be enabled at service level and disabled for specific proxy or request by mean of proxy/request options.
The url metrics label can be set by request options (default is request url) -> this is to prevent too many labels values for url when calling a parameterized url (e.g: /riders/:id).

related pr

Checklist

• your branch will not cause merge conflict with master
• run npm test
• tests are included
• the documentation is updated or integrated accordingly with your changes
• the code will follow the lint rules and style of the project
• you are not committing extraneous files or sensible data

[coveralls]

Coverage: 94.882% (+0.05%) from 94.828% when pulling 609b2b5 on feature/http-call-duration-metrics into fc31921 on master.

[silversoul93]

@fredmaggiowski is this branch mergeable?