[REFACTOR] OTLP translator: simplify time conversion

We don't need multiple levels of abstraction to convert nanoseconds to
milliseconds.
We do benefit from tests, however.

Signed-off-by: Bryan Boreham <bjboreham@gmail.com>

storage/remote/otlptranslator/prometheusremotewrite/helper.go

@@ -24,7 +24,6 @@ import (
 	"slices"
 	"sort"
 	"strconv"
-	"time"
 	"unicode/utf8"
 
 	"github.com/cespare/xxhash/v2"
@@ -594,5 +593,5 @@ func addResourceTargetInfo(resource pcommon.Resource, settings Settings, timesta
 
 // convertTimeStamp converts OTLP timestamp in ns to timestamp in ms
 func convertTimeStamp(timestamp pcommon.Timestamp) int64 {
-	return timestamp.AsTime().UnixNano() / (int64(time.Millisecond) / int64(time.Nanosecond))
+	return int64(timestamp) / 1_000_000
 }

storage/remote/otlptranslator/prometheusremotewrite/helper_test.go

@@ -14,6 +14,7 @@ package prometheusremotewrite
 
 import (
 	"testing"
+	"time"
 
 	"github.com/stretchr/testify/assert"
 	"go.opentelemetry.io/collector/pdata/pcommon"
@@ -159,3 +160,21 @@ func TestCreateAttributes(t *testing.T) {
 		})
 	}
 }
+
+func Test_convertTimeStamp(t *testing.T) {
+	tests := []struct {
+		name string
+		arg  pcommon.Timestamp
+		want int64
+	}{
+		{"zero", 0, 0},
+		{"1ms", 1_000_000, 1},
+		{"1s", pcommon.Timestamp(time.Unix(1, 0).UnixNano()), 1000},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := convertTimeStamp(tt.arg)
+			assert.Equal(t, tt.want, got)
+		})
+	}
+}