Fix prometheus metrics text exporter rendering

Iterate through all text chunks that need to be emitted as
part of the metric text label section before rendering the
value and timestamp.

Author: adam-rl

src/textexporter.h

@@ -118,7 +118,10 @@ struct PrometheusMetric {
                 buf += view.size();
                 space -= view.size();
             }
+            _render<typename Split::Next>(buf, space, val);
+            return;
         }
+
         auto v = std::to_string(val);
         auto ts = std::to_string(photon::now / 1000);
         if (v.size() < space) {