PrometheusHighQueryLoad alert: use configured selector
Currently we're hardcoding `job="prometheus-k8s"` as selector. This
doesn't work if your prometheus is elsewhere. Fortunately we have
`prometheusSelector` in `$._config`  which all the other alerts use.
Use that here too.
{
alert: 'PrometheusHighQueryLoad',
expr: |||
- avg_over_time(prometheus_engine_queries{job="prometheus-k8s"}[5m]) / max_over_time(prometheus_engine_queries_concurrent_max{job="prometheus-k8s"}[5m]) > 0.8
+ avg_over_time(prometheus_engine_queries{%(prometheusSelector)s}[5m]) / max_over_time(prometheus_engine_queries_concurrent_max{%(prometheusSelector)s}[5m]) > 0.8
||| % $._config,
'for': '15m',
labels: {
