pgsql-exporter: correct metric type
Created by: danieldides
Hopefully fixes: https://github.com/sourcegraph/sourcegraph/issues/27387.
Tested in dogfood. After updating the image to my local version, the following metric shows on the pgsql/metrics
endpoint:
# HELP pg_sg_migration_status Whether the migration applied successfully
# TYPE pg_sg_migration_status gauge
pg_sg_migration_status{server="localhost:5432"} 0