I was trying to create a Diagnostics Client in the tsdb package, but
IIRC importing `monitor` caused an import cycle of:
tsdb -> monitor -> cluster -> tsdb.
Moving Diagnostics to its own package will allow further use of
diagnostics.Client without running into import cycles.
This change adds support for diagnostics by decomposing the existing
interface into two interfaces -- one for stats, and the other for
diags. It also adds some basic monitor of system, network, and the Go
runtime.
Mark Rushakoff
Philip O'Toole