RFC: Experimental profiling (#10375)

#### Description

This is an RFC discussing the proposed solution for adding the profiling
signal.
Follows
#10207

PoC:
#10307

cc @open-telemetry/profiling-triagers
@open-telemetry/profiling-approvers
@open-telemetry/profiling-maintainers
cc @open-telemetry/collector-approvers

---------

Co-authored-by: Christos Kalkanis <[email protected]>
Co-authored-by: Pablo Baeyens <[email protected]>

docs/rfcs/experimental-profiling.md

@@ -0,0 +1,108 @@
+# Experimental support for profiling
+
+## Overview
+
+The OpenTelemetry collector has traces, metrics and logs as stable signals. We
+want to start experimenting with support for profiles as an experimental
+signal. But we also don't want to introduce breaking changes in packages
+otherwise considered stable.
+
+This document describes:
+
+* The approach we intend to take to introduce profiling with no breaking changes
+* How the migration will happen once profiling goes stable
+
+### Discarded approaches
+
+#### Refactor everything into per-signal subpackages
+
+A first approach, discussed in [issue
+10207](https://github.com/open-telemetry/opentelemetry-collector/issues/10207)
+has been discarded.
+It aimed to refactor the current packages with per-signal subpackages, so each
+subpackage could have its own stability level, like pdata does.
+
+This has been discarded, as the Collector SIG does not want the profiling
+signal to impact the road to the collector reaching 1.0.
+
+#### Use build tags
+
+An approach would have been to use build tags to limit the availability of
+profiles within packages.
+
+This approach would make the UX very bad though, as most packages are meant to
+be imported and not used in a compiled collector. It would therefore not have
+been possible to specify the appropriate build tags.
+
+This has been discarded, as the usage would have been too difficult.
+
+## Proposed approach
+
+The proposed approach will consist of two main phases:
+
+* Introduce `experimental` packages for each required module of the collector that needs to be profiles-aware.
+	* `consumer`, `receiver`, `connector`, `component`, `processor`
+* Mark specific APIs as `experimental` in their godoc for parts that can't be a new package.
+	* `service`
+
+### Introduce "experimental" subpackages
+
+Each package that needs to be profiling signal-aware will have its public
+methods and interfaces moves into an internal subpackage.
+
+Then, the original package will get similar API methods and interfaces as the
+ones currently available on the main branch.
+
+The profiling methods and interfaces will be made available in a `profiles`
+subpackage.
+
+See [PR
+#10253](https://github.com/open-telemetry/opentelemetry-collector/pull/10253)
+for an example.
+
+### Mark specific APIs as `experimental`
+
+In order to boot a functional collector with profiles support, some stable
+packages need to be aware of the experimental ones.
+
+To support that case, we will mark new APIs as `experimental` with go docs.
+Every experimental API will be documented as such:
+
+```golang
+// # Experimental
+//
+// Notice: This method is EXPERIMENTAL and may be changed or removed in a
+// later release.
+```
+
+As documented, APIs marked as experimental may changed or removed across
+releases, without it being considered as a breaking change.
+
+There are no symbols that would need to be marked as experimental today. If
+there ever are then implementers may add an experimental comment to them
+
+#### User specified configuration
+
+The user-specified configuration will let users specify a `profiles` pipeline:
+
+```
+service:
+	pipelines:
+		profiles:
+			receivers: [otlp]
+			processors: [batch]
+			exporters: [otlp]
+```
+
+When an experimental signal is being used, the collector will log a warning at
+boot.
+
+## Signal status change
+
+If the profiling signal becomes stable, all the experimental packages will be
+merged back into their stable counterpart, and the `service` module's imports
+will be updated.
+
+If the profiling signal is removed, all the experimental packages will be
+removed from the repository, and support for them will be removed in the
+`service` module.