Guard hot-path log calls to avoid payload rendering when disabled

Wrap `log.transport()` in `Channel.send()` and `log.runtime()` in
`PldRx.decode_pld()` with `log.at_least_level()` checks so that
expensive `pformat(payload)` / `repr(msg)` / `repr(pld)` calls are
skipped entirely when the respective log level is not active.

Previously the f-string arguments were eagerly evaluated before being
passed to the log method, even though `StackLevelAdapter.log()` would
then discard the message internally via its own `isEnabledFor()` check.
On high-frequency IPC paths this caused `pformat` to dominate CPU
usage (~60-70 %) as reported in #455.

This also restores the full diagnostic output (msg type, decoded
payload) that was temporarily commented out in 0373164 as a stopgap.

Resolves #455

tractor/ipc/_chan.py

@@ -325,10 +325,12 @@ class Channel:
         '''
         __tracebackhide__: bool = hide_tb
         try:
-            log.transport(
-                '=> send IPC msg\n\n'
-                # f'{pformat(payload)}\n'
-            )
+            if log.at_least_level('transport'):
+                # don't materialize the payload repr if not necessary
+                log.transport(
+                    '=> send IPC msg:\n\n'
+                    f'{pformat(payload)}\n'
+                )
             # assert self._transport  # but why typing?
             await self._transport.send(
                 payload,

tractor/msg/_ops.py

@@ -306,15 +306,15 @@ class PldRx(Struct):
             ):
                 try:
                     pld: PayloadT = self._pld_dec.decode(pld)
-                    log.runtime(
-                        f'Decoded payload for\n'
-                        # # f'\n'
-                        # f'{msg}\n'
-                        # # ^TODO?, ideally just render with `,
-                        # # pld={decode}` in the `msg.pformat()`??
-                        # f'where, '
-                        # f'{type(msg).__name__}.pld={pld!r}\n'
-                    )
+                    if log.at_least_level('runtime'):
+                        # don't materialize the payload repr if not necessary
+                        log.runtime(
+                            f'Decoded payload for\n'
+                            f'\n'
+                            f'{msg}\n'
+                            f'where, '
+                            f'{type(msg).__name__}.pld={pld!r}\n'
+                        )
                     return pld
                 except TypeError as typerr:
                     __tracebackhide__: bool = False