Telemetry integration.

Every function takes the event NAMESPACE as its first argument — the atom an emitted event name begins with, as in [namespace, event, :start].

It is an argument rather than a constant because a :telemetry handler subscribes to an event NAME and to nothing else. Two services emitting the same name cannot be told apart by a handler, whatever it inspects, because both events are delivered to every handler attached to that name. A host running more than one service therefore has to be able to give each service its own namespace, or it cannot report on them separately — only on all of them at once, with a guess about which is which.

A caller that needs no separation passes :jsonrpc2 and gets the names this library has always emitted.