Adding examples + doc

Author: leandrodamascena

docs/core/metrics.md

@@ -131,6 +131,19 @@ If you'd like to remove them at some point, you can use `clear_default_dimension
     --8<-- "examples/metrics/src/set_default_dimensions_log_metrics.py"
     ```
 
+### Changing default timestamp
+
+When creating metrics, we use the current timestamp. If you want to change the timestamp of all the metrics you create, utilize the `set_timestamp` function.
+
+???+ info
+    If you need to use different timestamps across multiple metrics, opt for [single_metric](#working-with-different-timestamp).
+
+=== "set_custom_timestamp_log_metrics.py"
+
+    ```python hl_lines="15"
+    --8<-- "examples/metrics/src/set_custom_timestamp_log_metrics.py"
+    ```
+
 ### Flushing metrics
 
 As you finish adding all your metrics, you need to serialize and flush them to standard output. You can do that automatically with the `log_metrics` decorator.

examples/metrics/src/set_custom_timestamp_log_metrics.py

@@ -0,0 +1,15 @@
+import datetime
+
+from aws_lambda_powertools import Metrics
+from aws_lambda_powertools.metrics import MetricUnit
+from aws_lambda_powertools.utilities.typing import LambdaContext
+
+metrics = Metrics()
+
+
+@metrics.log_metrics  # ensures metrics are flushed upon request completion/failure
+def lambda_handler(event: dict, context: LambdaContext):
+    metrics.add_metric(name="SuccessfulBooking", unit=MetricUnit.Count, value=1)
+
+    metric_timestamp = int((datetime.datetime.now() - datetime.timedelta(days=2)).timestamp() * 1000)
+    metrics.set_timestamp(metric_timestamp)