Add script to generate time series

The text import format supports specifying a time stamp [1]. Use this to
generate a time series for thing_count and thing_failures.

    ./things-series  > http/metrics
    podman pod rm -f learn-metrics || true
    podman play kube learn-metrics.yaml

First non-trivial query from "indefinitely long lookback" counters: "How
many tests happened in the last 5 minutes?

    rate(thing_count{job="host-http"}[5m]) * 5 * 60

The re-scaling is because the natural unit is "1/s" and we want the unit
to be the same as the original counter.

[1] https://prometheus.io/docs/instrumenting/exposition_formats/