dockerversion: remove insertUpstreamUserAgent()

It was not really "inserting" anything, just formatting and appending.
Simplify this by changing this in to a `getUpstreamUserAgent()` function
which returns the upstream User-Agent (if any) into a `UpstreamClient()`.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

dockerversion/useragent.go

@@ -18,11 +18,11 @@ type UAStringKey struct{}
 //
 //	[docker client's UA] UpstreamClient([upstream client's UA])
 func DockerUserAgent(ctx context.Context) string {
-	daemonUA := getDaemonUserAgent()
-	if upstreamUA := getUserAgentFromContext(ctx); len(upstreamUA) > 0 {
-		return insertUpstreamUserAgent(upstreamUA, daemonUA)
+	ua := getDaemonUserAgent()
+	if upstreamUA := getUpstreamUserAgent(ctx); upstreamUA != "" {
+		ua += " " + upstreamUA
 	}
-	return daemonUA
+	return ua
 }
 
 var (
@@ -57,16 +57,23 @@ func getDaemonUserAgent() string {
 	return daemonUA
 }
 
-// getUserAgentFromContext returns the previously saved user-agent context stored in ctx, if one exists
-func getUserAgentFromContext(ctx context.Context) string {
+// getUpstreamUserAgent returns the previously saved user-agent context stored
+// in ctx, if one exists, and formats it as:
+//
+//	UpstreamClient(<upstream user agent string>)
+//
+// It returns an empty string if no user-agent is present in the context.
+func getUpstreamUserAgent(ctx context.Context) string {
 	var upstreamUA string
 	if ctx != nil {
-		var ki interface{} = ctx.Value(UAStringKey{})
-		if ki != nil {
+		if ki := ctx.Value(UAStringKey{}); ki != nil {
 			upstreamUA = ctx.Value(UAStringKey{}).(string)
 		}
 	}
-	return upstreamUA
+	if upstreamUA == "" {
+		return ""
+	}
+	return fmt.Sprintf("UpstreamClient(%s)", escapeStr(upstreamUA))
 }
 
 const charsToEscape = `();\`
@@ -89,11 +96,3 @@ func escapeStr(s string) string {
 	}
 	return ret
 }
-
-// insertUpstreamUserAgent adds the upstream client useragent to create a user-agent
-// string of the form:
-//
-//	$dockerUA UpstreamClient($upstreamUA)
-func insertUpstreamUserAgent(upstreamUA string, dockerUA string) string {
-	return fmt.Sprintf("%s UpstreamClient(%s)", dockerUA, escapeStr(upstreamUA))
-}