Add a dashboard in a new metrics.rs module (#48)

* Add a dashboard in a new metrics.rs momdule

This change introduces a simple HTML table for quick reference about
individual symbol status. The table is served using a new HTTP
endpoint, under "/dashboard".

* Cargo.toml: remove premature lazy_static and prometheus deps

* metrics.rs: fix warnings and errors

* dashbrd: tidy uptime formatting, explain recursion limit and git dep

* mestrics.rs: detect and warn about duplicate symbol names

* agent.rs: Reunite stray import with friends and family

(they lived happily ever after)

* agent.rs, metrics.rs: Add metrics server configuration options

Author: drozdziak1