[confmap] Add featuregate to use stable expansion rules

.chloggen/confmap-add-expand-featuregate.yaml

@@ -0,0 +1,29 @@
+# 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: confmap
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Add `otelcol.unifyEnvVarExpansion` feature gate to allow enabling Collector/Configuration SIG environment variable expansion rules.
+
+# One or more tracking issues or pull requests related to the change
+issues: [10259]
+
+# (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: |
+  When enabled, this feature gate will:
+    - Disable expansion of BASH-style env vars (`$FOO`)
+    - `${FOO}` will be expanded as if it was `${env:FOO}
+  See https://github.com/open-telemetry/opentelemetry-collector/blob/main/docs/rfcs/env-vars.md for more details.
+
+# 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: []

cmd/builder/internal/builder/templates/go.mod.tmpl

@@ -7,6 +7,7 @@ go 1.21
 require (
 	{{if .Distribution.SupportsConfmapFactories -}}
 	go.opentelemetry.io/collector/confmap/converter/expandconverter v{{.Distribution.OtelColVersion}}
+	go.opentelemetry.io/collector/featuregate v{{.Distribution.OtelColVersion}}
 	{{- range .Providers}}
 	{{if .GoMod}}{{.GoMod}}{{end}}
 	{{- end}}

cmd/builder/internal/builder/templates/main.go.tmpl

@@ -10,13 +10,21 @@ import (
 	{{- if .Distribution.SupportsConfmapFactories}}
 	"go.opentelemetry.io/collector/confmap"
 	"go.opentelemetry.io/collector/confmap/converter/expandconverter"
+	"go.opentelemetry.io/collector/featuregate"
 	{{- range .Providers}}
 	{{.Name}} "{{.Import}}"
 	{{- end}}
 	{{- end}}
 	"go.opentelemetry.io/collector/otelcol"
 )
 
+{{ if .Distribution.SupportsConfmapFactories }}
+var useUnifiedEnvVarExpansionRules = featuregate.GlobalRegistry().MustRegister("otelcol.unifyEnvVarExpansion",
+	featuregate.StageAlpha,
+	featuregate.WithRegisterFromVersion("v0.102.0"),
+	featuregate.WithRegisterDescription("`${FOO}` will now be expanded as if it was `${env:FOO}` and no longer expands $ENV syntax. See https://github.com/open-telemetry/opentelemetry-collector/blob/main/docs/rfcs/env-vars.md for more details."))
+{{- end}}
+
 func main() {
 	info := component.BuildInfo{
 		Command:     "{{ .Distribution.Name }}",
@@ -38,14 +46,21 @@ func main() {
 				{{- if .ConfResolver.DefaultURIScheme }}
 				DefaultScheme: "{{ .ConfResolver.DefaultURIScheme }}",
 				{{- end }}
-				ConverterFactories: []confmap.ConverterFactory{
-					expandconverter.NewFactory(),
-				},
 			},
 		},
 		{{- end}}
 	}
 
+    {{ if .Distribution.SupportsConfmapFactories }}
+    if useUnifiedEnvVarExpansionRules.IsEnabled() && set.ConfigProviderSettings.ResolverSettings.DefaultScheme == "" {
+        set.ConfigProviderSettings.ResolverSettings.DefaultScheme = "env"
+    } else {
+        set.ConfigProviderSettings.ResolverSettings.ConverterFactories = []confmap.ConverterFactory{
+            expandconverter.NewFactory(),
+        }
+    }
+    {{- end}}
+
 	if err := run(set); err != nil {
 		log.Fatal(err)
 	}

cmd/otelcorecol/go.mod

@@ -27,6 +27,7 @@ require (
 	go.opentelemetry.io/collector/extension/ballastextension v0.102.1
 	go.opentelemetry.io/collector/extension/memorylimiterextension v0.102.1
 	go.opentelemetry.io/collector/extension/zpagesextension v0.102.1
+	go.opentelemetry.io/collector/featuregate v1.9.0
 	go.opentelemetry.io/collector/otelcol v0.102.1
 	go.opentelemetry.io/collector/processor v0.102.1
 	go.opentelemetry.io/collector/processor/batchprocessor v0.102.1
@@ -92,7 +93,6 @@ require (
 	go.opentelemetry.io/collector/config/internal v0.102.1 // indirect
 	go.opentelemetry.io/collector/consumer v0.102.1 // indirect
 	go.opentelemetry.io/collector/extension/auth v0.102.1 // indirect
-	go.opentelemetry.io/collector/featuregate v1.9.0 // indirect
 	go.opentelemetry.io/collector/pdata v1.9.0 // indirect
 	go.opentelemetry.io/collector/semconv v0.102.1 // indirect
 	go.opentelemetry.io/collector/service v0.102.1 // indirect