[configoptional] Mark 'configoptional.AddEnabledField' as beta (#14136)

<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->
#### Description

<!-- Issue number if applicable -->

Marks `configoptional.AddEnabledField` as beta. I have verified that it
would work correctly for cookies configuration and made a PoC for
keepalives (I will push a PR for it after this PR has been merged).

I spent some time updating
open-telemetry/opentelemetry-specification/issues/4344 and filed
open-telemetry/opentelemetry-specification/pull/4723 as a way to
explicitly state that the guidance only applies to environment
variables. The issue remains unresolved, but given the current usages of
`enabled` (see
open-telemetry/opentelemetry-specification#4344 (comment))
I don't see a reason to not go forward with this.

#### Link to tracking issue
Updates #14021

Author: mx-psi

.chloggen/mx-psi_mark-configoptional-gate-as-beta.yaml

@@ -0,0 +1,25 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: enhancement
+
+# The name of the component, or a single word describing the area of concern, (e.g. otlpreceiver)
+component: pkg/config/configoptional
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Mark `configoptional.AddEnabledField` as beta
+
+# One or more tracking issues or pull requests related to the change
+issues: [14021]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:
+
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: []

config/configoptional/optional.go

@@ -171,7 +171,7 @@ var (
 	addEnabledFieldFeatureGateID = "configoptional.AddEnabledField"
 	addEnabledFieldFeatureGate   = featuregate.GlobalRegistry().MustRegister(
 		addEnabledFieldFeatureGateID,
-		featuregate.StageAlpha,
+		featuregate.StageBeta,
 		featuregate.WithRegisterFromVersion("v0.138.0"),
 		featuregate.WithRegisterDescription("Allows optional fields to be toggled via an 'enabled' field."),
 		featuregate.WithRegisterReferenceURL("https://github.com/open-telemetry/opentelemetry-collector/issues/14021"),