Add gauge subtype for prometheus unknown type

opentelemetry/proto/metrics/v1/metrics.proto

@@ -201,6 +201,15 @@ message Metric {
 // "StartTimeUnixNano" is ignored for all data points.
 message Gauge {
   repeated NumberDataPoint data_points = 1;
+
+  // type provides additional type information for gauges.
+  // 
+  // When receiving from older producers of OTLP, it is safe to assume the
+  // zero-value for type, GAUGE_TYPE_STANDARD. When sending OTLP to older
+  // consumers, it is safe for them to ignore the field. In both cases, the
+  // previous behavior without type is maintained until producer and
+  // consumer are aware of the field.
+  GaugeType type = 2;
 }
 
 // Sum represents the type of a scalar metric that is calculated as a sum of all
@@ -337,6 +346,24 @@ enum DataPointFlags {
   // Bits 2-31 are reserved for future use.
 }
 
+// GaugeType defines valid gauge "sub-types", which are used to describe
+// gauges which have additional properties. These types generally apply
+// to metrics which originate from protocols which support these additional
+// types, but are represented as gauges in OTLP.
+enum GaugeType {
+  // GAUGE_TYPE_STANDARD is the default GaugeType, which represents a standard
+  // gauge. Since the OpenTelemetry API enforces types, this type is used for
+  // all metrics from OpenTelemetry instrumentation.
+  GAUGE_TYPE_STANDARD = 0;
+
+  // GAUGE_TYPE_UNKNOWN is a GaugeType which represents a metric for which the
+  // real type is not known, but is treated as a Gauge, such as a Prometheus
+  // metric without a TYPE comment. For example, the metric may be a
+  // monotonically-increasing sum, or histogram, but type information was not
+  // included at the source.
+  GAUGE_TYPE_UNKNOWN = 1;
+}
+
 // NumberDataPoint is a single data point in a timeseries that describes the
 // time-varying scalar value of a metric.
 message NumberDataPoint {