Telemetry instrumentation for Modbuv.Tcp.Client and Modbuv.Rtu.Master
requests.
Each request emits [:modbuv, :request, :start] when it is sent and
[:modbuv, :request, :stop] when it completes, fails, or times out.
Start measurements are %{system_time: native}. Stop measurements are
%{duration: native, monotonic_time: native}; convert duration with
System.convert_time_unit/3 to get RTT.
Metadata contains :transport (:tcp or :rtu), :role (:client or
:master), :mode (:passive or :active), :slave, and :fc. The stop
event adds :result (:ok or :error) and :reason (nil on success).