fix(options): merge RunTags across config layers instead of replacing

internal/cmd/config_consolidation_test.go

@@ -359,15 +359,18 @@ func getConfigConsolidationTestCases() []configConsolidationTestCase {
 			},
 		},
 
-		// Test-wide Tags
+		// Test-wide Tags: CLI and file tags are merged; CLI wins on key collision
 		{
 			opts{
 				fs:  defaultConfig(`{"tags": { "codeTagKey": "codeTagValue"}}`),
 				cli: []string{"--tag", "clitagkey=clitagvalue"},
 			},
 			exp{},
 			func(t *testing.T, c Config) {
-				exp := map[string]string{"clitagkey": "clitagvalue"}
+				exp := map[string]string{
+					"codeTagKey": "codeTagValue",
+					"clitagkey":  "clitagvalue",
+				}
 				assert.Equal(t, exp, c.RunTags)
 			},
 		},

lib/options.go

@@ -7,6 +7,7 @@ import (
 	"encoding/pem"
 	"errors"
 	"fmt"
+	"maps"
 	"net"
 	"reflect"
 	"slices"
@@ -490,7 +491,15 @@ func (o Options) Apply(opts Options) Options {
 		o.SystemTags = opts.SystemTags
 	}
 	if len(opts.RunTags) > 0 {
-		o.RunTags = opts.RunTags
+		// Tags are merged across config layers rather than replaced wholesale,
+		// so that each layer can contribute its own keys. When the same key
+		// appears in multiple layers the higher-priority layer wins, consistent
+		// with the order of precedence documented at:
+		// https://grafana.com/docs/k6/latest/using-k6/k6-options/how-to/#order-of-precedence
+		merged := make(map[string]string, len(o.RunTags)+len(opts.RunTags))
+		maps.Copy(merged, o.RunTags)
+		maps.Copy(merged, opts.RunTags)
+		o.RunTags = merged
 	}
 	if opts.MetricSamplesBufferSize.Valid {
 		o.MetricSamplesBufferSize = opts.MetricSamplesBufferSize

lib/options_test.go

@@ -550,6 +550,18 @@ func TestOptions(t *testing.T) {
 		opts := Options{}.Apply(Options{RunTags: tags})
 		assert.Equal(t, tags, opts.RunTags)
 	})
+	t.Run("RunTagsMerge", func(t *testing.T) {
+		t.Parallel()
+		base := Options{RunTags: map[string]string{"env": "staging", "team": "backend"}}
+		// higher-priority layer adds a new key and overrides an existing one
+		override := Options{RunTags: map[string]string{"env": "prod", "region": "us-east"}}
+		opts := base.Apply(override)
+		assert.Equal(t, map[string]string{
+			"env":    "prod",    // higher-priority layer wins on collision
+			"team":   "backend", // lower-priority key preserved
+			"region": "us-east", // higher-priority new key added
+		}, opts.RunTags)
+	})
 	t.Run("DiscardResponseBodies", func(t *testing.T) {
 		t.Parallel()
 		opts := Options{}.Apply(Options{DiscardResponseBodies: null.BoolFrom(true)})