This vendors the Prometheus client code, and exports metrics on the debug port, under `/metrics`.
This will currently export just the default metrics, like `go_goroutines`, `process_cpu_seconds_total`, `process_open_fds`, and `process_resident_memory_bytes`. Later work will start exporting Boulder-specific metrics, but this will allow Ops to start configuring scraping of Prometheus metrics in production.
Tests pass:
```
$ git diff master Godeps/ | sed -ne 's/^+.*ImportPath": "//p' | tr -d '",' | xargs go test
ok github.com/beorn7/perks/quantile 0.562s
ok github.com/matttproud/golang_protobuf_extensions/pbutil 0.003s
ok github.com/prometheus/client_golang/prometheus 34.418s
ok github.com/prometheus/client_golang/prometheus/promhttp 0.003s
? github.com/prometheus/client_model/go [no test files]
ok github.com/prometheus/common/expfmt 0.019s
ok github.com/prometheus/common/internal/bitbucket.org/ww/goautoneg 0.002s
ok github.com/prometheus/common/model 0.003s
ok github.com/prometheus/procfs 0.008s
```
Part of #2284
Jacob Hoffman-Andrews