[promises] increase debuggability of loop (#38137)

Add a couple of traces to aid debugging loops in the future

Closes #38137

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/38137 from ctiller:flake-fightas-28 648538f5e7
PiperOrigin-RevId: 697291436

src/core/BUILD

@@ -894,6 +894,7 @@ grpc_cc_library(
         "poll",
         "promise_factory",
         "//:gpr_platform",
+        "//:grpc_trace",
     ],
 )
 

src/core/lib/promise/loop.h

@@ -22,6 +22,7 @@
 #include "absl/status/status.h"
 #include "absl/status/statusor.h"
 #include "absl/types/variant.h"
+#include "src/core/lib/debug/trace.h"
 #include "src/core/lib/promise/detail/promise_factory.h"
 #include "src/core/lib/promise/poll.h"
 #include "src/core/util/construct_destruct.h"
@@ -100,6 +101,8 @@ class Loop {
   Loop& operator=(const Loop& loop) = delete;
 
   GPR_ATTRIBUTE_ALWAYS_INLINE_FUNCTION Poll<Result> operator()() {
+    GRPC_TRACE_LOG(promise_primitives, INFO)
+        << "loop[" << this << "] begin poll started=" << started_;
     if (!started_) {
       started_ = true;
       Construct(&promise_, factory_.Make());
@@ -113,14 +116,20 @@ class Loop {
         //  from our factory.
         auto lc = LoopTraits<PromiseResult>::ToLoopCtl(std::move(*p));
         if (absl::holds_alternative<Continue>(lc)) {
+          GRPC_TRACE_LOG(promise_primitives, INFO)
+              << "loop[" << this << "] iteration complete, continue";
           Destruct(&promise_);
           Construct(&promise_, factory_.Make());
           continue;
         }
+        GRPC_TRACE_LOG(promise_primitives, INFO)
+            << "loop[" << this << "] iteration complete, return";
         //  - otherwise there's our result... return it out.
         return absl::get<Result>(std::move(lc));
       } else {
         // Otherwise the inner promise was pending, so we are pending.
+        GRPC_TRACE_LOG(promise_primitives, INFO)
+            << "loop[" << this << "] pending";
         return Pending();
       }
     }