Represent SDM Counters as Gauges In Prometheus Exposition Format (#4101)

Co-authored-by: kkeirstead <[email protected]>

Author: github-actions[bot]

src/Microsoft.Diagnostics.Monitoring.WebApi/Metrics/MetricsStore.cs

@@ -200,8 +200,8 @@ private static string GetMetricType(EventType eventType)
         {
             switch (eventType)
             {
+                // In Prometheus, rates are treated as gauges due to their non-monotonic nature
                 case EventType.Rate:
-                    return "counter";
                 case EventType.Gauge:
                     return "gauge";
                 case EventType.Histogram:

src/Tests/Microsoft.Diagnostics.Monitoring.Tool.UnitTests/MetricsFormattingTests.cs

@@ -99,7 +99,7 @@ public async Task CounterFormat_Test()
 
             Assert.Equal(3, lines.Count);
             Assert.Equal($"# HELP {metricName}{payload.Unit} {payload.DisplayName}", lines[0]);
-            Assert.Equal($"# TYPE {metricName} counter", lines[1]);
+            Assert.Equal($"# TYPE {metricName} gauge", lines[1]);
             Assert.Equal($"{metricName} {payload.Value} {new DateTimeOffset(payload.Timestamp).ToUnixTimeMilliseconds()}", lines[2]);
         }