Fix ModelCheckpoint misformats filename

[bkiat1123]

What does this PR do?

As reported in this issue, filename format failed when one metric key is substring of another.

filename = "{epoch:03d}-{epoch_test:03d}"
groups = re.findall(r"(\{.*?)[:\}]", filename)
for group in groups:
    filename = filename.replace(group, ...)

In the first iteration of the for loop, both epoch and epoch_test will be formatted, which cause the error. Solve by reverse sorting the keys by length.

Fixes #16290

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