sdk_exporters/prometheus: Clarify Content Negotiation (#4543)

## Changes

Addressing feedback received during the last Spec SIG meeting, I'm
splitting my previous
PR(#4533)
into two to ensure we keep the scope of the original one for the
configuration changes.

If I understand correctly, almost all SDKs (if not all) are already
compliant since they are wrappers around Prometheus SDKs, and Prometheus
SDKs already do content negotiation properly. Then the PoCs are already
done :)

* [X] Related issues #4494 
* [ ] 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
* [ ]
[`spec-compliance-matrix.md`](https://github.com/open-telemetry/opentelemetry-specification/blob/main/spec-compliance-matrix.md)
updated if necessary

---------

Signed-off-by: Arthur Silva Sens <[email protected]>
Co-authored-by: Reiley Yang <[email protected]>
Co-authored-by: David Ashpole <[email protected]>

Author: 3 people

CHANGELOG.md

@@ -40,6 +40,9 @@ release.
 
 ### Compatibility
 
+- Clarify expectations about Prometheus content negotiation for metric names.
+  ([#4543](https://github.com/open-telemetry/opentelemetry-specification/pull/4543))
+
 ### SDK Configuration
 
 ### Common

specification/metrics/sdk_exporters/prometheus.md

@@ -74,3 +74,9 @@ The option MAY be named `without_scope_info`, and MUST be `false` by default.
 
 A Prometheus Exporter MAY support a configuration option to produce metrics without a [target info](../../compatibility/prometheus_and_openmetrics.md#resource-attributes-1)
 metric. The option MAY be named `without_target_info`, and MUST be `false` by default.
+
+## Content Negotiation
+
+A Prometheus Exporter MUST support content negotiation to allow clients to request
+metrics in different formats based on the `Accept` header in HTTP requests. Content
+negotiation MUST follow [Prometheus Content Negotiation guidelines](https://prometheus.io/docs/instrumenting/content_negotiation/).