[EventEngine] Change TXT lookup result type to std::vector<std::string> (#33030)

One TXT lookup query can return multiple TXT records (see the following
example). `EventEngine::DNSResolver` should return all of them to let
the caller (e.g. `event_engine_client_channel_resolver`) decide which
one they would use.

```
$ dig TXT wikipedia.org

; <<>> DiG 9.18.12-1+build1-Debian <<>> TXT wikipedia.org
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 49626
;; flags: qr rd ra; QUERY: 1, ANSWER: 3, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 512
;; QUESTION SECTION:
;wikipedia.org.                 IN      TXT

;; ANSWER SECTION:
wikipedia.org.          600     IN      TXT     "google-site-verification=AMHkgs-4ViEvIJf5znZle-BSE2EPNFqM1nDJGRyn2qk"
wikipedia.org.          600     IN      TXT     "yandex-verification: 35c08d23099dc863"
wikipedia.org.          600     IN      TXT     "v=spf1 include:wikimedia.org ~all"
```

Note that this change also deviates us from the iomgr's DNSResolver API
which uses std::string as the result type.


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include/grpc/event_engine/event_engine.h

@@ -354,7 +354,7 @@ class EventEngine : public std::enable_shared_from_this<EventEngine> {
         absl::AnyInvocable<void(absl::StatusOr<std::vector<SRVRecord>>)>;
     /// Called with the result of a TXT record lookup
     using LookupTXTCallback =
-        absl::AnyInvocable<void(absl::StatusOr<std::string>)>;
+        absl::AnyInvocable<void(absl::StatusOr<std::vector<std::string>>)>;
 
     virtual ~DNSResolver() = default;
 

src/core/ext/filters/client_channel/resolver/dns/event_engine/event_engine_client_channel_resolver.cc

@@ -31,6 +31,7 @@
 #include "absl/container/flat_hash_set.h"
 #include "absl/status/status.h"
 #include "absl/status/statusor.h"
+#include "absl/strings/match.h"
 #include "absl/strings/str_cat.h"
 #include "absl/strings/strip.h"
 #include "absl/types/optional.h"
@@ -128,7 +129,7 @@ class EventEngineClientChannelDNSResolver : public PollingResolver {
     void OnBalancerHostnamesResolved(
         std::string authority,
         absl::StatusOr<std::vector<EventEngine::ResolvedAddress>> addresses);
-    void OnTXTResolved(absl::StatusOr<std::string> service_config);
+    void OnTXTResolved(absl::StatusOr<std::vector<std::string>> service_config);
     // Returns a Result if resolution is complete.
     // callers must release the lock and call OnRequestComplete if a Result is
     // returned. This is because OnRequestComplete may Orphan the resolver,
@@ -251,7 +252,7 @@ EventEngineClientChannelDNSResolver::EventEngineDNSRequestWrapper::
         resolver_.get(), resolver_->name_to_resolve().c_str());
     txt_handle_ = event_engine_resolver_->LookupTXT(
         [self = Ref(DEBUG_LOCATION, "OnTXTResolved")](
-            absl::StatusOr<std::string> service_config) {
+            absl::StatusOr<std::vector<std::string>> service_config) {
           self->OnTXTResolved(std::move(service_config));
         },
         absl::StrCat("_grpc_config.", resolver_->name_to_resolve()),
@@ -388,7 +389,7 @@ void EventEngineClientChannelDNSResolver::EventEngineDNSRequestWrapper::
 }
 
 void EventEngineClientChannelDNSResolver::EventEngineDNSRequestWrapper::
-    OnTXTResolved(absl::StatusOr<std::string> service_config) {
+    OnTXTResolved(absl::StatusOr<std::vector<std::string>> service_config) {
   ValidationErrors::ScopedField field(&errors_, "txt lookup");
   absl::optional<Resolver::Result> result;
   {
@@ -400,7 +401,21 @@ void EventEngineClientChannelDNSResolver::EventEngineDNSRequestWrapper::
       errors_.AddError(service_config.status().message());
       service_config_json_ = service_config.status();
     } else {
-      service_config_json_ = absl::StrCat("grpc_config=", *service_config);
+      constexpr char kServiceConfigAttributePrefix[] = "grpc_config=";
+      auto result = std::find_if(service_config->begin(), service_config->end(),
+                                 [&](absl::string_view s) {
+                                   return absl::StartsWith(
+                                       s, kServiceConfigAttributePrefix);
+                                 });
+      if (result != service_config->end()) {
+        service_config_json_ =
+            result->substr(sizeof(kServiceConfigAttributePrefix));
+      } else {
+        service_config_json_ = absl::UnavailableError(absl::StrCat(
+            "failed to find attribute prefix: ", kServiceConfigAttributePrefix,
+            " in TXT records"));
+        errors_.AddError(service_config_json_.status().message());
+      }
     }
     result = OnResolvedLocked();
   }

test/core/event_engine/thready_event_engine/thready_event_engine.cc

@@ -144,7 +144,7 @@ ThreadyEventEngine::ThreadyDNSResolver::LookupTXT(LookupTXTCallback on_resolve,
                                                   Duration timeout) {
   return impl_->LookupTXT(
       [this, on_resolve = std::move(on_resolve)](
-          absl::StatusOr<std::string> record) mutable {
+          absl::StatusOr<std::vector<std::string>> record) mutable {
         return engine_->Asynchronously([on_resolve = std::move(on_resolve),
                                         record = std::move(record)]() mutable {
           on_resolve(std::move(record));