fix(httpd): Fixes key collisions when serializing /debug/vars

[stuartcarnie]

This PR fixes an issue with serveExpvar so that it does not generate invalid JSON. Specifically, the improvement was made to prevent key collisions when serializing []*monitor.Statistic values.

The algorithm works as such:

1. record each statistic key in a map with a count of occurrences
2. if the count > 0, amend key with the suffix :<count>

In practice, key collisions for hh_processor were observed with Enterprise:

  "hh:/var/lib/influxdb/hh": {"name":"hh",
  "hh_processor": {"name":"hh_processor","tags":{"db"
  "hh_processor": {"name":"hh_processor","tags":{"db"
  "hh_processor": {"name":"hh_processor","tags":{"db"
  "hh_processor": {"name":"hh_processor","tags":{"db"
  "hh_database": {"name":"hh_database","tags":{"db"

This change will serialize these values as:

  "hh:/var/lib/influxdb/hh": {"name":"hh",
  "hh_processor": {"name":"hh_processor","tags":{"db"
  "hh_processor:1": {"name":"hh_processor","tags":{"db"
  "hh_processor:2": {"name":"hh_processor","tags":{"db"
  "hh_processor:3": {"name":"hh_processor","tags":{"db"
  "hh_database": {"name":"hh_database","tags":{"db"

[dgnorton]

lgtm 👍