Clarify output values

Modify header message to make it more obvious what is being printed.
Print each value and the difference, for clarity.

python/lsst/verify/bin/print_metricvalues.py

@@ -72,7 +72,13 @@ def main():
     else:
         collection2 = args.collection2 if args.collection2 is not None else args.collection
         butler2 = lsst.daf.butler.Butler(args.repo2, collections=collection2)
-        print(f"Showing difference of {args.repo2}#{collection2} - {args.repo}#{args.collection}")
+        print("Printed values are `repo2 - repo1 = delta`, where:")
+        print(f"repo2 = {args.repo2}#{collection2}")
+        print(f"repo1 = {args.repo}#{args.collection}")
+        if args.repo2 == args.repo:
+            print(f"delta = ({collection2}) - ({args.collection})")
+        else:
+            print(f"delta = ({args.repo2}) - ({args.repo})")
         extract_metricvalues.print_diff_metrics(butler,
                                                 butler2,
                                                 data_id_keys=args.data_id_keys,

python/lsst/verify/extract_metricvalues.py

@@ -127,7 +127,7 @@ def print_diff_metrics(butler1, butler2, data_id_keys=None, verbose=False):
 
         delta = value2.quantity - value1.quantity
         if delta != 0 or verbose:
-            print(f"{value1.datum.label}: {delta} / {value1.quantity}")
+            print(f"{value1.datum.label}: {value2.quantity} - {value1.quantity} = {delta}")
         if delta == 0:
             same += 1
 

tests/test_extract_metricvalues.py

@@ -186,13 +186,13 @@ def check_stdout(n, last_line, **kwargs):
         result = check_stdout(12, last_line)
         expect = "{instrument: 'TestCam', detector: 12, visit: 12345, ...}"
         self.assertIn(expect, result)
-        expect = "verify.another: 1.0 mas / 3.0 mas"
+        expect = "verify.another: 4.0 mas - 3.0 mas = 1.0 mas"
         self.assertIn(expect, result)
 
         result = check_stdout(12, last_line, data_id_keys=("detector", "visit"))
         expect = "detector: 12, visit: 12345"
         self.assertIn(expect, result)
-        expect = "verify.another: 1.0 mas / 3.0 mas"
+        expect = "verify.another: 4.0 mas - 3.0 mas = 1.0 mas"
         self.assertIn(expect, result)