Add Dagster exporter to list of miscellaneous exporters

[HirofumiTsuda]

cc @RichiH

Re-submission of #3051, which was closed with the feedback that the exporter
had "limited community" and "seemingly low adoption" at the time, with an
invitation to try again later. Adoption has grown since then: 11 stars, 1
fork (the repo was ~1 week old with no real track record when #3051 was
filed), and it now ships with a Grafana dashboard alongside the exporter.

Adds the Dagster exporter
to the Miscellaneous section, in alphabetical order (between c-lightning
exporter and DHCPD leases exporter).

It polls Dagster's GraphQL API directly and exposes run-state metrics (active
runs, completed runs, latest run status) as Prometheus metrics, avoiding the
blind spots of Dagster's official Pushgateway-based integration (e.g.
crashed/queued runs that never get a chance to push).

Sample output:

dagster_active_runs{job_name="heavy_job",location="dev-dagster-workspace",status="queued"} 0
dagster_active_runs{job_name="heavy_job",location="dev-dagster-workspace",status="started"} 1
dagster_active_runs{job_name="heavy_job",location="dev-dagster-workspace",status="starting"} 0

dagster_completed_runs_total{job_name="heavy_job",location="dev-dagster-workspace",status="failure"} 0
dagster_completed_runs_total{job_name="heavy_job",location="dev-dagster-workspace",status="success"} 12
dagster_completed_runs_total{job_name="failing_job",location="dev-dagster-workspace",status="failure"} 3

dagster_last_run_info{job_name="heavy_job",location="dev-dagster-workspace",status="success"} 1
dagster_last_run_info{job_name="failing_job",location="dev-dagster-workspace",status="failure"} 1

License: MIT