Stabilize OTLP->Prometheus: Gauges and Sums (#5034)

Fixes #4919 
Fixes #4920

## Changes

Stabilizes the converstion of OTLP Gauges and Sums to their Prometheus
counterparts.

For non-trivial changes, follow the [change proposal
process](https://github.com/open-telemetry/opentelemetry-specification/blob/main/CONTRIBUTING.md#proposing-a-change).

* [X] Related issues #4919, #4920
* [ ] Related [OTEP(s)](https://github.com/open-telemetry/oteps) #
* [ ] Links to the prototypes (when adding or changing features)
* [x]
[`CHANGELOG.md`](https://github.com/open-telemetry/opentelemetry-specification/blob/main/CHANGELOG.md)
file updated for non-trivial changes
* For trivial changes, include `[chore]` in the PR title to skip the
changelog check
* [ ] [Spec compliance
matrix](https://github.com/open-telemetry/opentelemetry-specification/blob/main/spec-compliance-matrix/template.yaml)
updated if necessary
* [ ] [Declarative config data
model](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/configuration/data-model.md#overview)
is updated if SDK config surface is changed

---------

Signed-off-by: Arthur Silva Sens <arthursens2005@gmail.com>
Co-authored-by: Jack Berg <34418638+jack-berg@users.noreply.github.com>

Author: ArthurSens

CHANGELOG.md

@@ -35,6 +35,10 @@ release.
 
 ### Compatibility
 
+- Stabilize sections of Prometheus and OpenMetrics Compatibility.
+  - Stabilize OpenTelemetry Gauge and Sum to Prometheus transformations.
+    ([#5034](https://github.com/open-telemetry/opentelemetry-specification/pull/5034))
+
 ### SDK Configuration
 
 ### Supplementary Guidelines

specification/compatibility/prometheus_and_openmetrics.md

@@ -466,34 +466,32 @@ to avoid conflicts with the already existing `otel_scope_name`, `otel_scope_vers
 
 ### Gauges
 
-**Status**: [Development](../document-status.md)
+**Status**: [Stable](../document-status.md)
 
-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][metricMetadata] is `unknown`. Otherwise, it MUST be converted
-to a Prometheus Gauge.
+An [OpenTelemetry Gauge](../metrics/data-model#gauge) MUST be converted following 
+a hint present in [metric.metadata][metricMetadata]:
+- If the `prometheus.type` key is absent, or its value is equal to `gauge`, the datapoint MUST be transformed to a Prometheus Gauge.
+- If the `prometheus.type` key has value equal to `unkown`, the datapoint MUST be transformed to a Prometheus Unknown.
+- If the `prometheus.type` key has value equal to `info`, the datapoint SHOULD be transformed to a Prometheus Info.
+- If the `prometheus.type` key has value equal to `stateset`, the datapoint SHOULD be transformed to a Prometheus Stateset.
 
 Exemplars on OpenTelemetry Gauges SHOULD be dropped.
 
 ### Sums
 
-**Status**: [Development](../document-status.md)
+**Status**: [Stable](../document-status.md)
 
-[OpenTelemetry Sums](../metrics/data-model.md#sums) follows this logic:
+An [OpenTelemetry Sum](../metrics/data-model.md#sums) MUST be converted following the rules below:
 
 - 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][metricMetadata] 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][metricMetadata] 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:
+- If the aggregation temporality is cumulative and the sum is non-monotonic, it should follow the same rules as described for [OpenTelemetry Gauge](#gauges-1)
+- If the aggregation temporality is delta and the sum is monotonic, it MAY 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.
   - The new data point's start time must match the time of the accumulated data point. If not, see [detecting alignment issues](../metrics/data-model.md#sums-detecting-alignment-issues).
-- Otherwise, it MUST be dropped.
 
-If the metric name for monotonic Sum metric points does not end in a suffix of `_total` a suffix of `_total` SHOULD be added by default, otherwise the name MUST remain unchanged. Exporters SHOULD provide a configuration option to disable the addition of `_total` suffixes.
-Monotonic Sum metric points with `StartTimeUnixNano` should export the `{name}_created` metric as well.
+If the metric name for monotonic Sum metric points does not end in a suffix of `_total` a suffix of `_total` SHOULD be added by default, otherwise the name MUST remain unchanged.
+
+Monotonic Sum metric points with `StartTimeUnixNano` SHOULD transform `StartTimeUnixNano` into Prometheus `StartTime`, following the appropriate format used by each Prometheus protocol. 
 
 If Sum is converted to a Prometheus Counter, then `Exemplars` MUST be converted
 as described in the [Exemplar Conversion](#exemplar-conversion) section.