[RLS] Change case of RLS 'x-google-rls-data' header to lowercase. (#32760)

"X-Google-RLS-Data" does not work as gRPC metadata key.

src/core/ext/filters/client_channel/lb_policy/rls/rls.cc

@@ -117,7 +117,7 @@ constexpr absl::string_view kRls = "rls_experimental";
 const char kGrpc[] = "grpc";
 const char* kRlsRequestPath = "/grpc.lookup.v1.RouteLookupService/RouteLookup";
 const char* kFakeTargetFieldValue = "fake_target_field_value";
-const char* kRlsHeaderKey = "X-Google-RLS-Data";
+const char* kRlsHeaderKey = "x-google-rls-data";
 
 const Duration kDefaultLookupServiceTimeout = Duration::Seconds(10);
 const Duration kMaxMaxAge = Duration::Minutes(5);

test/cpp/end2end/rls_end2end_test.cc

@@ -101,7 +101,7 @@ class MyTestServiceImpl : public BackendService {
                     ::testing::Pair(kCallCredsMdKey, kCallCredsMdValue)));
     IncreaseRequestCount();
     auto client_metadata = context->client_metadata();
-    auto range = client_metadata.equal_range("X-Google-RLS-Data");
+    auto range = client_metadata.equal_range("x-google-rls-data");
     {
       grpc::internal::MutexLock lock(&mu_);
       for (auto it = range.first; it != range.second; ++it) {