fix(metrics): fix race condition in metrics.Init()

IterationMetricsEnabled can be concurrently set

```
==================
WARNING: DATA RACE
Write at 0x00c000124098 by goroutine 12:
  github.com/form3tech-oss/f1/v2/internal/metrics.Init()
      /Users/nikolayvladimirov/go/src/github.com/form3tech-oss/f1/internal/metrics/metrics.go:82 +0x7c
  github.com/form3tech-oss/f1/v2/internal/metrics_test.TestMetrics_Init_IsSafe.func1()
      /Users/nikolayvladimirov/go/src/github.com/form3tech-oss/f1/internal/metrics/metrics_test.go:15 +0x20

Previous write at 0x00c000124098 by goroutine 14:
  github.com/form3tech-oss/f1/v2/internal/metrics.Init()
      /Users/nikolayvladimirov/go/src/github.com/form3tech-oss/f1/internal/metrics/metrics.go:82 +0x7c
  github.com/form3tech-oss/f1/v2/internal/metrics_test.TestMetrics_Init_IsSafe.func1()
      /Users/nikolayvladimirov/go/src/github.com/form3tech-oss/f1/internal/metrics/metrics_test.go:15 +0x20

Goroutine 12 (running) created at:
  github.com/form3tech-oss/f1/v2/internal/metrics_test.TestMetrics_Init_IsSafe()
      /Users/nikolayvladimirov/go/src/github.com/form3tech-oss/f1/internal/metrics/metrics_test.go:14 +0xa4
  testing.tRunner()
      /opt/homebrew/Cellar/go/1.23.1/libexec/src/testing/testing.go:1690 +0x184
  testing.(*T).Run.gowrap1()
      /opt/homebrew/Cellar/go/1.23.1/libexec/src/testing/testing.go:1743 +0x40

Goroutine 14 (finished) created at:
  github.com/form3tech-oss/f1/v2/internal/metrics_test.TestMetrics_Init_IsSafe()
      /Users/nikolayvladimirov/go/src/github.com/form3tech-oss/f1/internal/metrics/metrics_test.go:14 +0xa4
  testing.tRunner()
      /opt/homebrew/Cellar/go/1.23.1/libexec/src/testing/testing.go:1690 +0x184
  testing.(*T).Run.gowrap1()
      /opt/homebrew/Cellar/go/1.23.1/libexec/src/testing/testing.go:1743 +0x40
```

internal/metrics/metrics.go

@@ -79,7 +79,6 @@ func Init(iterationMetricsEnabled bool) {
 		}
 		m = NewInstance(defaultRegistry, iterationMetricsEnabled)
 	})
-	m.IterationMetricsEnabled = iterationMetricsEnabled
 }
 
 func Instance() *Metrics {

internal/metrics/metrics_test.go

@@ -0,0 +1,21 @@
+package metrics_test
+
+import (
+	"testing"
+
+	"github.com/form3tech-oss/f1/v2/internal/metrics"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestMetrics_Init_IsSafe(t *testing.T) {
+	metrics.Init(true)
+
+	// race detector assertion
+	for range 10 {
+		go func() {
+			metrics.Init(false)
+		}()
+	}
+
+	assert.True(t, metrics.Instance().IterationMetricsEnabled)
+}

pkg/f1/f1_scenarios_test.go

@@ -16,3 +16,18 @@ func TestCombineScenarios(t *testing.T) {
 	then.
 		each_scenarios_setup_and_iteration_functions_are_called()
 }
+
+func TestCombineScenarios_1(t *testing.T) {
+	t.Parallel()
+
+	given, when, then := newF1ScenarioStage(t)
+
+	given.
+		f1_is_configured_to_run_a_combined_scenario()
+
+	when.
+		the_f1_scenario_is_executed()
+
+	then.
+		each_scenarios_setup_and_iteration_functions_are_called()
+}