In most formatters, any unused keys in the args dict will just be
discarded. Because JSONFormatter logged the entire dict in addition
to the message, some values were included in the output that may
not have been intended. This could include sensitive data, so we
should stop that.
In the interest of maintaining compatibility with any tools that are
reading the args dict, we leave the dict but filter out any unused
keys.
Change-Id: Ib64837c1ae93a27bef3d30a776320a373f18dd1c
Closes-Bug: 1571714
Closes-Bug: 1787214