Add TODO in prometheus exporter

src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/PrometheusSerializerExt.cs

@@ -99,7 +99,7 @@ public static int WriteMetric(byte[] buffer, int cursor, Metric metric)
                     buffer[cursor++] = ASCII_LINEFEED;
                 }
             }
-            else
+            else if (metric.MetricType.IsHistogram())
             {
                 foreach (ref readonly var metricPoint in metric.GetMetricPoints())
                 {
@@ -194,6 +194,10 @@ public static int WriteMetric(byte[] buffer, int cursor, Metric metric)
                     buffer[cursor++] = ASCII_LINEFEED;
                 }
             }
+            else
+            {
+                // TODO: Support exponential histogram.
+            }
 
             buffer[cursor++] = ASCII_LINEFEED;