[chore] add nometrics build tagging to metrics API endpoint (#3331)

* add nometrics API endpoint code, to ensure prometheus NEVER compiled in unless wanted

* whoops, fix build tagging

internal/api/metrics/metrics.go

@@ -15,6 +15,8 @@
 // You should have received a copy of the GNU Affero General Public License
 // along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
+//go:build !nometrics
+
 package metrics
 
 import (