Add a span to 'create_neon_superuser' for better OpenTelemetry traces (#6644)

create_neon_superuser runs the first queries in the database after cold
start. Traces suggest that those first queries can make up a significant
fraction of the cold start time. Make it more visible by adding an
explict tracing span to it; currently you just have to deduce it by
looking at the time spent in the parent 'apply_config' span subtracted
by all the other child spans.

compute_tools/src/compute.rs

@@ -207,6 +207,7 @@ fn maybe_cgexec(cmd: &str) -> Command {
 
 /// Create special neon_superuser role, that's a slightly nerfed version of a real superuser
 /// that we give to customers
+#[instrument(skip_all)]
 fn create_neon_superuser(spec: &ComputeSpec, client: &mut Client) -> Result<()> {
     let roles = spec
         .cluster