fix(version): don't add "v" prefix to non-semver VERSION strings

When VERSION is set to a non-tag value like "main" (from goreleaser or
ldflags), the init() function unconditionally prepended "v", producing
"vmain". Now only add the "v" prefix when doing so produces a valid
semver string, leaving branch names and other non-semver values as-is.

version/version.go

@@ -90,10 +90,13 @@ func init() {
 		VERSION = "dev-snapshot"
 	} else {
 		if !strings.HasPrefix(VERSION, "v") {
-			VERSION = "v" + VERSION
+			vVersion := "v" + VERSION
+			if semver.IsValid(vVersion) {
+				VERSION = vVersion
+			}
 		}
-		if !semver.IsValid(VERSION) {
-			slog.Default().Warn("invalid version number", "version", VERSION)
+		if !semver.IsValid(VERSION) && VERSION != "dev-snapshot" {
+			slog.Default().Info("non-semver version", "version", VERSION)
 		}
 	}
 	if bi, ok := debug.ReadBuildInfo(); ok {

version/version_test.go

@@ -7,6 +7,7 @@ import (
 
 	"github.com/prometheus/client_golang/prometheus"
 	dto "github.com/prometheus/client_model/go"
+	"golang.org/x/mod/semver"
 )
 
 func TestCheckVersion(t *testing.T) {
@@ -48,9 +49,12 @@ func TestVersionInfo(t *testing.T) {
 		t.Error("VersionInfo().Version should not be empty")
 	}
 
-	// Version should start with "v" or be "dev-snapshot"
+	// Version should start with "v", be "dev-snapshot", or be a non-semver string (e.g. branch name)
 	if !strings.HasPrefix(info.Version, "v") && info.Version != "dev-snapshot" {
-		t.Errorf("Version should start with 'v' or be 'dev-snapshot', got: %s", info.Version)
+		// Non-semver versions like branch names ("main") are acceptable
+		if semver.IsValid("v" + info.Version) {
+			t.Errorf("Semver-like version should start with 'v', got: %s", info.Version)
+		}
 	}
 
 	// GitRevShort should be <= 7 characters if set
@@ -398,10 +402,10 @@ func TestParseBuildTimeConsistency(t *testing.T) {
 
 func BenchmarkParseBuildTime(b *testing.B) {
 	inputs := []string{
-		"1672531200",                 // Unix epoch
-		"2023-01-01T00:00:00Z",      // RFC3339
-		"invalid-timestamp",          // Invalid
-		"",                          // Empty
+		"1672531200",           // Unix epoch
+		"2023-01-01T00:00:00Z", // RFC3339
+		"invalid-timestamp",    // Invalid
+		"",                     // Empty
 	}
 
 	for _, input := range inputs {