[Exporter.Prometheus] Address feedback

- Fix escaping behaviour for unit suffixes.
- Fix incorrect label escaping for `:`.

Author: martincostello

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

@@ -17,13 +17,12 @@ public PrometheusMetric(string name, string unit, PrometheusType type, bool disa
         // consecutive `_` characters MUST be replaced with a single `_` character.
         // https://github.com/open-telemetry/opentelemetry-specification/blob/b2f923fb1650dde1f061507908b834035506a796/specification/compatibility/prometheus_and_openmetrics.md#L230-L233
         var sanitizedName = SanitizeMetricName(name);
-        var openMetricsName = EscapeOpenMetricsName(RemoveOpenMetricsCounterNameSuffix(name));
+        var openMetricsName = RemoveOpenMetricsCounterNameSuffix(name);
 
         string? sanitizedUnit = null;
         if (!string.IsNullOrEmpty(unit))
         {
             sanitizedUnit = GetUnit(unit);
-            var openMetricsUnitSuffix = EscapeOpenMetricsName(sanitizedUnit);
 
             // The resulting unit SHOULD be added to the metric as
             // [OpenMetrics UNIT metadata](https://github.com/prometheus/OpenMetrics/blob/v1.0.0/specification/OpenMetrics.md#metricfamily)
@@ -32,10 +31,12 @@ public PrometheusMetric(string name, string unit, PrometheusType type, bool disa
             if (!sanitizedName.EndsWith(sanitizedUnit, StringComparison.Ordinal))
             {
                 sanitizedName += $"_{sanitizedUnit}";
-                openMetricsName += $"_{openMetricsUnitSuffix}";
+                openMetricsName += $"_{sanitizedUnit}";
             }
         }
 
+        openMetricsName = EscapeOpenMetricsName(openMetricsName);
+
         // If the metric name for monotonic Sum metric points does not end in a suffix of `_total` a suffix of `_total` MUST be added by default, otherwise the name MUST remain unchanged.
         // Exporters SHOULD provide a configuration option to disable the addition of `_total` suffixes.
         // https://github.com/open-telemetry/opentelemetry-specification/blob/b2f923fb1650dde1f061507908b834035506a796/specification/compatibility/prometheus_and_openmetrics.md#L286

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

@@ -527,12 +527,12 @@ public static int WriteTargetInfo(byte[] buffer, int cursor, Resource resource,
     }
 
     private static int WritePrometheusLabelKey(byte[] buffer, int cursor, string value)
-        => WriteNormalizedLabelKey(buffer, cursor, value, isOpenMetrics: false);
+        => WriteNormalizedLabelKey(buffer, cursor, value);
 
     private static int WriteOpenMetricsLabelKey(byte[] buffer, int cursor, string value)
-        => WriteNormalizedLabelKey(buffer, cursor, value, isOpenMetrics: true);
+        => WriteNormalizedLabelKey(buffer, cursor, value);
 
-    private static int WriteNormalizedLabelKey(byte[] buffer, int cursor, string value, bool isOpenMetrics)
+    private static int WriteNormalizedLabelKey(byte[] buffer, int cursor, string value)
     {
         var lastCharUnderscore = false;
 
@@ -549,7 +549,7 @@ private static int WriteNormalizedLabelKey(byte[] buffer, int cursor, string val
                 }
             }
 
-            if (!IsAllowedMetricsLabelCharacter(ch, isOpenMetrics))
+            if (!IsAllowedMetricsLabelCharacter(ch))
             {
                 if (!lastCharUnderscore)
                 {
@@ -568,8 +568,8 @@ private static int WriteNormalizedLabelKey(byte[] buffer, int cursor, string val
     }
 
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
-    private static bool IsAllowedMetricsLabelCharacter(char value, bool isOpenMetrics) =>
-        char.IsAsciiLetterOrDigit(value) || value is '_' || (isOpenMetrics && value == ':');
+    private static bool IsAllowedMetricsLabelCharacter(char value) =>
+        char.IsAsciiLetterOrDigit(value) || value is '_';
 
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
     private static int WriteUnicodeScalar(byte[] buffer, int cursor, string value, ref int index)

test/OpenTelemetry.Exporter.Prometheus.HttpListener.Tests/PrometheusMetricTests.cs

@@ -168,6 +168,10 @@ public void OpenMetricsName_CollapsesConsecutiveUnderscores()
     public void OpenMetricsName_PreserveLeadingNumber()
         => AssertOpenMetricsName("2_metric_name", "By", PrometheusType.Gauge, false, "_2_metric_name_bytes");
 
+    [Fact]
+    public void OpenMetricsName_UnitStartingWithNumber_DoesNotAddExtraSeparator()
+        => AssertOpenMetricsName("metric", "2", PrometheusType.Gauge, false, "metric_2");
+
     [Fact]
     public void OpenMetricsName_CollapsesConsecutiveUnsupportedCharacters()
         => AssertOpenMetricsName("s%%ple", "%/m", PrometheusType.Summary, false, "s_ple_percent_per_minute");

test/OpenTelemetry.Exporter.Prometheus.HttpListener.Tests/PrometheusSerializerTests.cs

@@ -293,13 +293,13 @@ public void WriteLabelKeyPrefixesLeadingDigits(bool openMetricsRequested)
     }
 
     [Fact]
-    public void WriteLabelKeyPreservesOpenMetricsLegacyValidCharacters()
+    public void WriteLabelKeyNormalizesOpenMetricsLabelNames()
     {
         var buffer = new byte[32];
 
         var cursor = PrometheusSerializer.WriteLabelKey(buffer, 0, "a_b:c", openMetricsRequested: true);
 
-        Assert.Equal("a_b:c", Encoding.UTF8.GetString(buffer, 0, cursor));
+        Assert.Equal("a_b_c", Encoding.UTF8.GetString(buffer, 0, cursor));
     }
 
     [Fact]