Avoid _doing_it_wrong() for Server-Timing in Optimization Detective when output buffering is not enabled

[westonruter]

When Optimization Detective active, I noticed a _doing_it_wrong() warning in Query Monitor:

In the Server-Timing admin page, output buffering is not enabled:

The issue is that Performance Lab's Server-Timing functionality is not aware that Optimization Detective is doing output buffering of the template (although it used to be in #875, but it was backed out in #1082). So the PL functionality is sending the Server-Timing header after Optimization Detective starts buffering the output, but then when the buffer closes and the server timing wrapper attempts to then process the timing, it fails because the header has already been sent. So we need to add an additional check for whether the output buffering checkbox has been checked (via the perflab_server_timing_use_output_buffer() function).

(If Performance Lab had been aware of the Optimization Detective output buffer, then it wouldn't need the checkbox at all since the Server-Timing header could be sent after the output buffer is closed.)

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Co-authored-by: westonruter <[email protected]>
Co-authored-by: swissspidy <[email protected]>

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