decode(
line) ->msg,err
Decode one protocol line into a tagged message table
Accepts a line with or without the trailing newline (\r\n is
tolerated for interactive debugging). Returns a table tagged by
kind: "metric", "block", "query", "ok", "err", "qd" or "qe".
Metric labels stay in their raw canonical string form; the daemon
treats them as an opaque, sorted identity component.