Prometheus compatibility: represent Prometheus Info, StateSet and Unk…

…nown-typed metrics in OTLP

CHANGELOG.md

@@ -33,6 +33,9 @@ release.
 
 ### Compatibility
 
+- Prometheus: represent Prometheus Info, StateSet and Unknown-typed metrics in OTLP.
+  ([#3868](https://github.com/open-telemetry/opentelemetry-specification/pull/3868))
+
 ### SDK Configuration
 
 - Add file configuration section to spec compliance matrix.

specification/compatibility/prometheus_and_openmetrics.md

@@ -72,6 +72,8 @@ The [OpenMetrics TYPE metadata](https://github.com/OpenObservability/OpenMetrics
 if present, MUST be used to determine the OTLP data type, and dictates
 type-specific conversion rules listed below. Metric families without type
 metadata follow rules for [unknown-typed](#unknown-typed) metrics below.
+The TYPE metadata MUST also be added to the OTLP [metric.metadata](https://github.com/open-telemetry/opentelemetry-proto/blob/c451441d7b73f702d1647574c730daf7786f188c/opentelemetry/proto/metrics/v1/metrics.proto#L199)
+under the `prometheus.type` key (e.g. `prometheus.type="unknown"`).
 
 ### Counters
 
@@ -269,13 +271,22 @@ Prometheus exporters SHOULD provide a configuration option to disable the
 
 ### Gauges
 
-An [OpenTelemetry Gauge](../metrics/data-model.md#gauge) MUST be converted to a Prometheus Gauge.
+An [OpenTelemetry Gauge](../metrics/data-model.md#gauge) MUST be converted to
+a Prometheus Unknown-typed metric if the `prometheus.type` key of
+[metric.metadata](https://github.com/open-telemetry/opentelemetry-proto/blob/c451441d7b73f702d1647574c730daf7786f188c/opentelemetry/proto/metrics/v1/metrics.proto#L199)
+is `unknown`. Otherwise, it MUST be converted to a Prometheus Gauge.
 
 ### Sums
 
 [OpenTelemetry Sums](../metrics/data-model.md#sums) follows this logic:
 
 - If the aggregation temporality is cumulative and the sum is monotonic, it MUST be converted to a Prometheus Counter.
+- If the aggregation temporality is cumulative and the sum is non-monotonic and the `prometheus.type` key of
+[metric.metadata](https://github.com/open-telemetry/opentelemetry-proto/blob/c451441d7b73f702d1647574c730daf7786f188c/opentelemetry/proto/metrics/v1/metrics.proto#L199)
+is `info`, it MUST be converted to an OpenMetrics Info metric.
+- If the aggregation temporality is cumulative and the sum is non-monotonic and the `prometheus.type` key of
+[metric.metadata](https://github.com/open-telemetry/opentelemetry-proto/blob/c451441d7b73f702d1647574c730daf7786f188c/opentelemetry/proto/metrics/v1/metrics.proto#L199)
+is `stateset`, it MUST be converted to an OpenMetrics StateSet metric.
 - If the aggregation temporality is cumulative and the sum is non-monotonic, it MUST be converted to a Prometheus Gauge.
 - If the aggregation temporality is delta and the sum is monotonic, it SHOULD be converted to a cumulative temporality and become a Prometheus Counter. The following behaviors are expected:
   - The new data point type must be the same as the accumulated data point type.