[hpack] Include context when failing due to metadata size exceeded (#31809)

* better error

* Automated change: Fix sanity tests

* Update hpack_parser.cc

* fix

* reduce-bloat

* Automated change: Fix sanity tests

Co-authored-by: ctiller <ctiller@users.noreply.github.com>

src/core/ext/transport/chttp2/transport/hpack_parser.cc

@@ -1188,17 +1188,49 @@ class HPackParser::Parser {
         std::move(result));
   }
 
+  class MetadataSizeLimitExceededEncoder {
+   public:
+    explicit MetadataSizeLimitExceededEncoder(std::string& summary)
+        : summary_(summary) {}
+
+    void Encode(const Slice& key, const Slice& value) {
+      AddToSummary(key.as_string_view(), value.size());
+    }
+
+    template <typename Key, typename Value>
+    void Encode(Key, const Value& value) {
+      AddToSummary(Key::key(), Key::Encode(value).size());
+    }
+
+   private:
+    void AddToSummary(absl::string_view key,
+                      size_t value_length) GPR_ATTRIBUTE_NOINLINE {
+      absl::StrAppend(&summary_, " ", key, ":",
+                      hpack_constants::SizeForEntry(key.size(), value_length),
+                      "B");
+    }
+    std::string& summary_;
+  };
+
   GPR_ATTRIBUTE_NOINLINE
-  bool HandleMetadataSizeLimitExceeded(const HPackTable::Memento&) {
+  bool HandleMetadataSizeLimitExceeded(const HPackTable::Memento& md) {
+    // Collect a summary of sizes so far for debugging
+    // Do not collect contents, for fear of exposing PII.
+    std::string summary;
+    if (metadata_buffer_ != nullptr) {
+      MetadataSizeLimitExceededEncoder encoder(summary);
+      metadata_buffer_->Encode(&encoder);
+    }
+    summary =
+        absl::StrCat("; adding ", md.key(), " (length ", md.transport_size(),
+                     "B)", summary.empty() ? "" : " to ", summary);
     if (metadata_buffer_ != nullptr) metadata_buffer_->Clear();
-    // TODO(alishananda): add debug log with metadata details
     return input_->MaybeSetErrorAndReturn(
-        [this] {
+        [this, summary = std::move(summary)] {
           return grpc_error_set_int(
-              GRPC_ERROR_CREATE(absl::StrFormat(
-                  "received initial metadata size exceeds limit (%" PRIu32
-                  " vs. %" PRIu32 ")",
-                  *frame_length_, metadata_size_limit_)),
+              GRPC_ERROR_CREATE(absl::StrCat(
+                  "received initial metadata size exceeds limit (",
+                  *frame_length_, " vs. ", metadata_size_limit_, ")", summary)),
               StatusIntProperty::kRpcStatus, GRPC_STATUS_RESOURCE_EXHAUSTED);
         },
         false);