Add zoekt probe

Description

Related to gitlab-org/gitlab#521933 (closed)

This MR adds two items:

• Adds the ability to create a gauge metric to the Prometheus metrics class
• Adds a new zoekt probe to report metrics for zoekt data from the database

The zoekt metrics were originally implemented in a CronWorker but that was proving to be inaccurate (see related issue).

Testing

bin/gitlab-exporter web -c config/gitlab-exporter.yml

load the database metrics: http://localhost:9168/database

ci_stale_builds 0.0
ci_unarchived_traces 0.0
gitlab_database_rows{query_name="soft_deleted_projects"} 0.0
gitlab_database_rows{query_name="orphaned_projects"} 0.0
gitlab_database_rows{query_name="uploads"} 56.0
gitlab_database_rows{query_name="users",admin="false",external="false",state="active",access_level="50"} 31.0
gitlab_database_rows{query_name="users",admin="false",external="false",state="active",access_level="40"} 6.0
gitlab_database_rows{query_name="users",admin="false",external="false",state="active",access_level="10"} 1.0
gitlab_database_rows{query_name="users",admin="true",external="false",state="active",access_level="50"} 1.0
gitlab_database_rows{query_name="users",admin="false",external="false",state="active",access_level="15"} 1.0
gitlab_database_rows{query_name="users",admin="false",external="false",state="active",access_level="30"} 4.0
gitlab_database_rows{query_name="users",admin="false",external="false",state="active",access_level="0"} 21.0
gitlab_database_rows{query_name="users",admin="false",external="false",state="active",access_level="20"} 5.0
gitlab_database_rows{query_name="projects",visibility_level="10",archived="false"} 2.0
gitlab_database_rows{query_name="projects",visibility_level="0",archived="false"} 16.0
gitlab_database_rows{query_name="projects",visibility_level="20",archived="false"} 13.0
gitlab_database_rows{query_name="projects",visibility_level="0",archived="true"} 1.0
# HELP search_zoekt_task_processing_queue_size Number of tasks waiting to be processed by Zoekt
# TYPE search_zoekt_task_processing_queue_size gauge
search_zoekt_task_processing_queue_size{node_name="tchu--20220722-T2FH0",node_id="1"} 0
search_zoekt_task_processing_queue_size{node_name="tchu--20220722-T2FH0",node_id="2"} 0