[chore][confma] minor refactor of enableMergeAppendOption feature gate (#12976)

@mx-psi @dmitryax While working on my RFC, I realized that we can
simplify the logic by moving the merge behavior into `conf.Merge`.
Currently, the feature gate check happens externally, and we choose the
merge option based on that.

This PR moves the check inside `conf.Merge`. By doing this
[converters](https://github.com/open-telemetry/opentelemetry-collector/blob/main/confmap/README.md)
can also make use of this feature.

There are no changes to any API signatures, so this doesn’t introduce
any breaking changes. Let me know what you think about this.

Author: VihasMakwana

confmap/confmap.go

@@ -173,6 +173,10 @@ func (l *Conf) IsSet(key string) bool {
 // Merge merges the input given configuration into the existing config.
 // Note that the given map may be modified.
 func (l *Conf) Merge(in *Conf) error {
+	if enableMergeAppendOption.IsEnabled() {
+		// only use MergeAppend when enableMergeAppendOption featuregate is enabled.
+		return l.mergeAppend(in)
+	}
 	return l.k.Merge(in.k)
 }
 

confmap/resolver.go

@@ -183,13 +183,8 @@ func (mr *Resolver) Resolve(ctx context.Context) (*Conf, error) {
 		if err != nil {
 			return nil, err
 		}
-		if enableMergeAppendOption.IsEnabled() {
-			// only use MergeAppend when enableMergeAppendOption featuregate is enabled.
-			err = retMap.mergeAppend(retCfgMap)
-		} else {
-			err = retMap.Merge(retCfgMap)
-		}
-		if err != nil {
+
+		if err := retMap.Merge(retCfgMap); err != nil {
 			return nil, err
 		}
 	}