metrics: diff: fix float precision formatting (#3609)

Texttable will try to detect the column type automatically, which
results in a low precision of the float that it is showing. We need to
disable that auto-formatting and just use `str(val)` instead.

dvc/command/metrics.py

@@ -121,6 +121,9 @@ def _show_diff(diff):
 
     table = Texttable()
 
+    # disable automatic formatting
+    table.set_cols_dtype(["t", "t", "t", "t"])
+
     # remove borders to make it easier for users to copy stuff
     table.set_chars(("", "", "", ""))
     table.set_deco(0)

tests/unit/command/test_metrics.py

@@ -119,3 +119,12 @@ def test_metrics_show(dvc, mocker):
         all_branches=True,
         all_commits=True,
     )
+
+
+def test_metrics_diff_prec():
+    assert _show_diff(
+        {"other.json": {"a.b": {"old": 0.0042, "new": 0.0043, "diff": 0.0001}}}
+    ) == (
+        "   Path      Metric   Value    Change\n"
+        "other.json   a.b      0.0043   0.0001"
+    )