logging: support output to stderr (#5896)

(part of the getpage benchmarking epic #5771)

The plan is to make the benchmarking tool log on stderr and emit results
as JSON on stdout. That way, the test suite can simply take captures
stdout and json.loads() it, while interactive users of the benchmarking
tool have a reasonable experience as well.

Existing logging users continue to print to stdout, so, this change
should be a no-op functionally and performance-wise.

storage_broker/src/bin/storage_broker.rs

@@ -434,6 +434,7 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
     logging::init(
         LogFormat::from_config(&args.log_format)?,
         logging::TracingErrorLayerEnablement::Disabled,
+        logging::Output::Stdout,
     )?;
     logging::replace_panic_hook_with_tracing_panic_hook().forget();
     // initialize sentry if SENTRY_DSN is provided