Avoid sending Server-Timing header when buffer is being cleaned

[westonruter]

This is a sub-PR of #1317. Merge it first.

As @felixarntz pointed out in #1317 (comment), when a template calls ob_clean() it can currently result in the Server-Timing header being sent prematurely. When the buffer is being cleaned, the output buffer callback should not send the header yet because no output is being printed, so it can continue waiting until the output is flushed.

This also fixes a bug with passing the argument names when they are missing from the $args passed to \Perflab_Server_Timing::register_metric(). The PR also improves escaping the resultant strings.

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Co-authored-by: westonruter <westonruter@git.wordpress.org>
Co-authored-by: felixarntz <flixos90@git.wordpress.org>

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[felixarntz]

Thanks @westonruter!