As the configuration for the exporters and alertmanager is unchanged
between the two major versions this patch tries to minimize
duplication while at the same time as there's no upgrade path from 1.x
to 2.x, it allows running the two services in parallel. See also #56037

This uses fewer lines of code and one less process.

Previously the prometheus.service file looked like:

  ExecStart=/nix/store/wjkhfw3xgkmavz1akkqir99w4lbqhak7-prometheus-1.8.2-bin/bin/prometheus -storage.local.path=/var/lib/prometheus/metrics \
    -config.file=/nix/store/zsnvzw51mk3n1cxjd0351bj39k1j6j27-prometheus.yml-check-config-checked \
    -web.listen-address=0.0.0.0:9090 \
    -alertmanager.notification-queue-capacity=10000 \
    -alertmanager.timeout=10s \
     \

  Restart=always

Now it's:

  ExecStart=/nix/store/wjkhfw3xgkmavz1akkqir99w4lbqhak7-prometheus-1.8.2-bin/bin/prometheus \
    -storage.local.path=/var/lib/prometheus/metrics \
    -config.file=/nix/store/zsnvzw51mk3n1cxjd0351bj39k1j6j27-prometheus.yml-check-config-checked \
    -web.listen-address=0.0.0.0:9090 \
    -alertmanager.notification-queue-capacity=10000 \
    -alertmanager.timeout=10s
  Restart=always

This is to ensure more backwards compatibility. Note this is not 100%
backwards compatible because we now require dataDir to begin with /var/lib/.

Add Prometheus 2 service in parallel with 1.x version (continuation)