feat: add `$flagd.timestamp` to json evaluator (#958)

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## This PR
<!-- add the description of the PR here -->

Adds "timestamp" to the json evaluation context.


### Related Issues
<!-- add here the GitHub issue that this PR resolves if applicable -->

Related to #851. I am not sure we want to say that it closes the issue
though.

---------

Signed-off-by: Craig Pastro <pastro.craig@gmail.com>
Co-authored-by: Michael Beemer <beeme1mr@users.noreply.github.com>
Co-authored-by: Todd Baert <todd.baert@dynatrace.com>

core/pkg/eval/json_evaluator.go

@@ -9,6 +9,7 @@ import (
 	"regexp"
 	"strconv"
 	"strings"
+	"time"
 
 	"github.com/diegoholiveira/jsonlogic/v3"
 	"github.com/open-feature/flagd/core/pkg/logger"
@@ -38,7 +39,8 @@ const (
 var regBrace *regexp.Regexp
 
 type flagdProperties struct {
-	FlagKey string `json:"flagKey"`
+	FlagKey   string `json:"flagKey"`
+	Timestamp int64  `json:"timestamp"`
 }
 
 func init() {
@@ -321,7 +323,8 @@ func (je *JSONEvaluator) evaluateVariant(reqID string, flagKey string, context m
 		}
 
 		context = je.setFlagdProperties(context, flagdProperties{
-			FlagKey: flagKey,
+			FlagKey:   flagKey,
+			Timestamp: time.Now().Unix(),
 		})
 
 		b, err := json.Marshal(context)

core/pkg/eval/json_evaluator_test.go

@@ -1248,4 +1248,36 @@ func TestFlagdAmbientProperties(t *testing.T) {
 			t.Fatalf("expected %s, got %s", model.TargetingMatchReason, reason)
 		}
 	})
+
+	t.Run("timestampIsInTheContext", func(t *testing.T) {
+		evaluator := eval.NewJSONEvaluator(logger.NewLogger(nil, false), store.NewFlags())
+
+		_, _, err := evaluator.SetState(sync.DataSync{FlagData: `{
+			"flags": {
+				"welcome-banner": {
+					"state": "ENABLED",
+					"variants": {
+						"true": true,
+						"false": false
+					},
+					"defaultVariant": "false",
+					"targeting": {
+						"<": [ 1696904426, { "var": "$flagd.timestamp" } ]
+					}
+				}
+			}
+		}`})
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		value, variant, reason, _, err := evaluator.ResolveBooleanValue(context.Background(), "default", "welcome-banner", nil)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		if !value || variant != "true" || reason != model.TargetingMatchReason {
+			t.Fatal("timestamp was not in the context")
+		}
+	})
 }