[preview] Add `preview --num-days=N` to extend preview time window.

In the future, we might teach the preview window computation to
consider `window_size` for relevant alerts such as those with `type =
"avg_diff"`.

opmon/cli.py

@@ -457,6 +457,13 @@ def backfill(
     metavar="YYYY-MM-DD",
     required=False,
 )
+@click.option(
+    "--num-days",
+    "--num-days",
+    type=int,
+    help="Number of days for which the project be analyzed. Default: 3",
+    default=3,
+)
 @click.option(
     "--slug",
     help="Experimenter or Normandy slug associated with the project to create a preview for",
@@ -478,6 +485,7 @@ def preview(
     derived_dataset_id,
     start_date,
     end_date,
+    num_days,
     slug,
     config_file,
     config_repos,
@@ -485,16 +493,17 @@ def preview(
 ):
     """Create a preview for a specific project based on a subset of data."""
     if start_date is None and end_date is None:
-        start_date = datetime.today() - timedelta(days=3)
+        start_date = datetime.today() - timedelta(days=num_days)
         end_date = datetime.today() - timedelta(days=1)
         start_date = pytz.utc.localize(start_date)
         end_date = pytz.utc.localize(end_date)
     elif start_date is None:
-        start_date = end_date - timedelta(days=3)
-        start_date = pytz.utc.localize(start_date)
-    elif end_date is None:
-        end_date = start_date + timedelta(days=3)
-        end_date = pytz.utc.localize(end_date)
+        start_date = end_date - timedelta(days=num_days)
+    else:
+        end_date = start_date + timedelta(days=num_days)
+
+    start_date = pytz.utc.localize(start_date)
+    end_date = pytz.utc.localize(end_date)
 
     table = bq_normalize_name(slug)