[soundwave] Keep only 4 months of data in soundwave study outputs

This prevents the size of the output csv from growing without bounds.
The limit on the input that DataStudio can take seems to be around
100 MiB.

Change-Id: Ib4656fe066686931135e2280e042fc832f1bb520
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1844974
Auto-Submit: Juan Antonio Navarro Pérez <perezju@chromium.org>
Commit-Queue: Sami Kyöstilä <skyostil@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Cr-Commit-Position: refs/heads/master@{#703268}

tools/perf/cli_tools/soundwave/studies/__init__.py

@@ -21,6 +21,11 @@ def PostProcess(df):
   df['timestamp'] = df.groupby(
       ['test_suite', 'bot', 'point_id'])['timestamp'].transform('min')
 
+  # Prevent the size of the output from growing without bounts. Limit for
+  # DataStudio input appears to be around 100MiB.
+  four_months_ago = pandas.Timestamp.utcnow() - pandas.DateOffset(months=4)
+  df = df[df['timestamp'] > four_months_ago.tz_convert(None)].copy()
+
   # We use all runs on the latest day for each quarter as reference.
   df['quarter'] = df['timestamp'].dt.to_period('Q')
   df['reference'] = df['timestamp'].dt.date == df.groupby(