Admin REST API: stats

Pasted from the [tech spec](https://docs.google.com/document/d/1X9fFzqBZzrSmx2d0NkmjDW2tc8ysbLQS9LaRQRxdJZU/edit#):

`GET /stats`
Output all server stats.

**Output Sample**
```

cache::key::helloworld_production_lds::rq_count: 4
cache::key::foobar_production_cds::rq_count: 3
controlplane::connections::cx_active::1
controlplane::connections::cx_connect_fail::0
controlplane::connections::cx_destroy::0
controlplane::connections::cx_total::1
controlplane::requests::rq_1xx::0
controlplane::requests::rq_2xx::2
controlplane::requests::rq_3xx::0
controlplane::requests::rq_4xx::0
controlplane::requests::rq_5xx::0
controlplane::requests::rq_active::2
controlplane::requests::rq_error::0
controlplane::requests::rq_success::0
controlplane::requests::rq_timeout::0
controlplane::requests::rq_total::2
controlplane:::hostname::mycontrolplane.lyft.net
downstream::connections::cx_active::7
downstream::connections::cx_connect_fail::0
downstream::connections::cx_total::7
downstream::requests::rq_1xx::0
downstream::requests::rq_2xx::7
downstream::requests::rq_3xx::0
downstream::requests::rq_4xx::0
downstream::requests::rq_5xx::0
downstream::requests::rq_active::7
downstream::requests::rq_error::0
downstream::requests::rq_success::0
downstream::requests::rq_timeout::0
downstream::requests::rq_total::7
server::live: 1
server::memory_allocated: 16802608
server::memory_heap_size: 34603008
server::total_connections: 239
server::uptime: 132786
server::version: 11541598
```