nix(loadtest): report percentage change

This reports the percentage change between the current head branch and
the main branch, which is exactly the number we'll want to make our
decisions on "success or fail" on.

CI failures will initially be reported for regressions of 5% or more on
an individual number.

nix/tools/loadtest.nix

@@ -274,12 +274,28 @@ let
         import sys
         import pandas as pd
 
+
+        def evaluate_change(df):
+            return ((df['head'] / df['main'] - 1) * 100) \
+              .map(lambda r: "{icon} {ratio:.1f} %".format(
+                ratio=r,
+                # Hardcoded failure threshold for CI is 5% here.
+                icon="" if r < 5 else ":x:"
+              ))
+
+
         pd.read_json(sys.stdin) \
           .rename(columns={'latency': sys.argv[1]}) \
           .set_index(sys.argv[1]) \
           .drop(['branch']) \
           .convert_dtypes() \
-          .to_markdown(sys.stdout, floatfmt='.1f')
+          .assign(change=evaluate_change) \
+          .to_markdown(
+            sys.stdout,
+            floatfmt='.1f',
+            colglobalalign='right',
+            colalign=('left',)
+          )
       '';