Add Dagster exporter to list of miscellaneous exporters

[HirofumiTsuda]

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
  Add Dagster exporter to list of miscellaneous exporters #3051 was filed)
• Actively maintained: Grafana dashboard, Docker image, and Helm chart
  included; a combined traces+metrics demo (via docker-compose) with a
  sibling OpenTelemetry tracing project
• ~100 unique visitors and ~100 unique cloners over the last two weeks alone

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

[HirofumiTsuda]

Closing in favor of a fresh PR from a new branch (same content) to avoid any confusion with the previously-closed #3051.