u/metrics: Add option to fflush after writing each metrics record.
This allows to use the metrics logging in "streaming mode" where records are written out with low delay, e.g., into a Unix fifo file / pipe for live consumption by some tracing or recording application.
XRT_METRICS_EARLY_FLUSH=true enables this "streaming mode".
For a hacky (ab)use case, see following Psychtoolbox commit to implement a feedback channel for XR frame presentation onset timestamping:
https://github.com/Psychtoolbox-3/Psychtoolbox-3/commit/b1d90739a4b1469c14a42ed0f952822856e94580