[bug][mdatagen] Auto-enable v1 metrics when legacy is enabled

.chloggen/mdatagen-fix-auto-enable-versioned-metrics.yaml

@@ -0,0 +1,28 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: 'bug_fix'
+
+# The name of the component, or a single word describing the area of concern, (e.g. receiver/otlp)
+component: cmd/mdatagen
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Auto-enable v1 metrics when legacy metric is enabled and v1 feature gate is on
+
+# One or more tracking issues or pull requests related to the change
+issues: [15650]
+
+# (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 the v1 feature gate is enabled and a legacy metric is enabled, the corresponding
+  v1 metric is now programmatically enabled so users don't need to add the v1 metric
+  to their config.
+
+# 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/mdatagen/internal/templates/metrics.go.tmpl

@@ -408,6 +408,15 @@ func NewMetricsBuilder(mbc MetricsBuilderConfig, settings {{ .Status.Class }}.Se
   {{- range $name, $metric := .Metrics }}
   {{- if $metric.Migration }}
   if {{ printf "%s" $metric.Migration.ThroughGates.EnableNew | publicVar }}FeatureGate.IsEnabled() {
+   {{- /* If a legacy metric is enabled, programmatically enable the v1 metric.
+   The v1 metric may be disabled in metadata.yaml; when the v1 feature gate
+   is on we need to enable it automatically so users don't have to update
+   their config. */}}
+    if mb.metric{{ $name.Render }}.config.Enabled && !mb.metric{{ $metric.Migration.To.Render }}.config.enabledSetByUser {
+      mb.metric{{ $metric.Migration.To.Render }}.config.Enabled = true
+      mb.metric{{ $metric.Migration.To.Render }}.data = pmetric.NewMetric()
+      mb.metric{{ $metric.Migration.To.Render }}.init()
+    }
     if mb.metric{{ $name.Render }}.config.Enabled && mb.metric{{ $metric.Migration.To.Render }}.config.Enabled {
       var disable bool
       if mb.metric{{ $name.Render }}.data.Type() != mb.metric{{ $metric.Migration.To.Render }}.data.Type() {