Auto merge of #19257 - asajeffrey:test-perf-include-date-in-csv, r=jdm

Include a YYYYMMDD date field in the raw test-perf CSV to make Google Data Studio happy

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AFAICT Google Data Studio needs a YYYMMDD date string in the raw data to be able to provide time-indexed graphs. `TODATE` is not converting unix timestamps to YYYYMMDD.

https://www.en.advertisercommunity.com/t5/Data-Studio/Can-Data-Studio-Date-Dimension-use-epoch-in-seconds-from/td-p/839649

This PR is a workaround, and includes a date field in the raw CSV.

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- [X] `./mach test-tidy` does not report any errors
- [X] These changes do not require tests because this is test infrastructure

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etc/ci/performance/runner.py

@@ -10,11 +10,15 @@ import itertools
 import json
 import os
 import subprocess
+from datetime import datetime
 from functools import partial
 from statistics import median, StatisticsError
 from urllib.parse import urlsplit, urlunsplit, urljoin
 
 
+DATE = datetime.now().strftime("%Y%m%d")
+
+
 def load_manifest(filename):
     with open(filename, 'r') as f:
         text = f.read()
@@ -169,6 +173,7 @@ def parse_log(log, testcase, url):
     # rather than the url.
     def set_testcase(timing, testcase=None):
         timing['testcase'] = testcase
+        timing['date'] = DATE
         return timing
 
     valid_timing_for_case = partial(valid_timing, url=url)
@@ -240,6 +245,7 @@ def save_result_json(results, filename, manifest, expected_runs, base):
 def save_result_csv(results, filename, manifest, expected_runs, base):
 
     fieldnames = [
+        'date',
         'testcase',
         'title',
         'connectEnd',