wiggle: generate a span that is present at all levels

The code I wrote here prior was incorrect: a span is present at the
level specified and below; previously I thought it was present at the
level specified and above. So, previously, a TRACE-level event inside
this span would be associated with the module and function name provided
here. Now all events inside this span should be associated with it.

crates/wiggle/generate/src/funcs.rs

@@ -69,8 +69,9 @@ pub fn define_func(
         ) -> Result<#abi_ret, #rt::Trap> {
             use std::convert::TryFrom as _;
 
+            // This span is present at all levels (ERROR and below)
             let _span = #rt::tracing::span!(
-                #rt::tracing::Level::TRACE,
+                #rt::tracing::Level::ERROR,
                 "wiggle abi",
                 module = #mod_name,
                 function = #func_name