Add Dagster exporter to list of miscellaneous exporters - #3118

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. The repo was about a week old when #3051 was
filed. Since then:

Adoption and community

• Used in production by at least two teams. One proposed the alerting rules
  that now ship with the Helm chart
  (#112).
• Two outside code contributors. The second added Dagster+ (Dagster Cloud)
  support and tested it against a large production Dagster+ deployment
  (about 24 code locations, thousands of assets)
  (#139).
• Regular releases (latest v0.6.0), a Helm chart on
  ArtifactHub,
  and a Grafana dashboard on
  grafana.com (ID 25815).

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, schedule, sensor,
asset, and code-location metrics, avoiding the blind spots of pushing metrics
from inside runs to a Pushgateway (e.g. crashed or 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