Fix remote_mirrors usage ping metric

What does this MR do?

Fix remote_mirrors usage ping data metric

Old broken query with redundant distinct

    :remote_mirrors: SELECT DISTINCT COUNT(DISTINCT "projects"."creator_id") FROM
      "projects" INNER JOIN "remote_mirrors" ON "remote_mirrors"."project_id" = "projects"."id"
      WHERE "remote_mirrors"."enabled" = TRUE

New query

    :remote_mirrors: SELECT COUNT(DISTINCT "projects"."creator_id") FROM "projects"
      INNER JOIN "remote_mirrors" ON "remote_mirrors"."project_id" = "projects"."id"
      WHERE "remote_mirrors"."enabled" = TRUE

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Related to #325803 (closed)