Generalize Metrics to include self documentation

[TristanCacqueray]

The work started with #792, and here are a list of things to do to complete the work:

• [ ] Break all-in-one queries to use individual metrics. For example, 1st review mean time is currently hard-coded in the getReviewStats query here: https://github.com/change-metrics/monocle/blob/f05cd10244e4e83f49906088150a486b5e60d98d/haskell/src/Monocle/Backend/Queries.hs#L1028-L1032 . Instead the metric should be defined with the new Metric data type.
• [ ] Improve the Metric data type to include a long description, e.g. how to interpret the value.
• [ ] Make the web interface get the query list and include tooltip on the graph with each value details
• [ ] Enable direct metric query, e.g. curl /api/metric/get/timeToMerge?q=...
• [ ] Enable trend query, e.g. curl /api/metric/trend/timeToMerge?q=...&period=2weeks which should output an histogram.