gpuledger v0.4.0

What the GPUs cost: counters of the seconds each GPU spends in each state and of each
job's GPU-seconds, held or idle; gpuledger report reading them back from Prometheus;
the waste in the dashboard and an alert on it. Every number checked through a real
Prometheus on real Nomad allocations in the matrix; the driver side, as before, only
against a fake nvidia-smi (GL-10, v0.5.0).

Upgrading from 0.3.0

• Nothing to change: the counters are new series, and report is a new command.
• serve --history (which the job specs set) keeps the counters across restarts; without
  it they start from zero at each start, which Prometheus reads as a counter reset.
• The dashboard has three new panels and the rules a new alert; re-import them to get
  them.

Added

• The cost in the dashboard and the rules: GPU-hours per job over the range, the idle
  share per job, GPUs per state from the counters; GPULedgerJobMostlyIdle (info) when
  a job with at least a GPU-hour reserved over a day left more than half of it idle.
  Unit-tested with promtool test rules, a mutated threshold checked to fail; every
  panel's query run against the real Prometheus in the matrix (GL-32).
• gpuledger report --prometheus URL --since 7d: GPU-hours per job — reserved, held,
  idle and the idle share, most idle first — and per node and state, from the counters
  in Prometheus; --json under the JSON contract; a bearer token by variable name
  (GL-31).
• Counters: gpuledger_gpu_state_seconds_total{state} per GPU (GL-29) and
  gpuledger_job_gpu_seconds_total{namespace,nomad_job,use} per job, use held or
  idle (GL-30) — GPU-hours and waste over any window with increase(). Kept in the
  --history file across restarts; gaps and partial reads accrue nothing. The Nomad
  matrix checks them through a real Prometheus on real reservations.