Make the grpc_metadataArray property private.

src/objective-c/GRPCClient/GRPCCall.h

@@ -58,7 +58,6 @@ extern id const kGRPCTrailersKey;
 @protocol GRPCRequestHeaders <NSObject>
 
 @property(nonatomic, readonly) NSUInteger count;
-@property(nonatomic, readonly) grpc_metadata *grpc_metadataArray;
 
 - (id)objectForKeyedSubscript:(NSString *)key;
 - (void)setObject:(id)obj forKeyedSubscript:(NSString *)key;

src/objective-c/GRPCClient/private/GRPCWrappedCall.h

@@ -35,7 +35,7 @@
 #include <grpc/grpc.h>
 
 #import "GRPCChannel.h"
-#import "GRPCCall.h"
+#import "GRPCRequestHeaders.h"
 
 @interface GRPCOperation : NSObject
 @property(nonatomic, readonly) grpc_op op;
@@ -45,7 +45,7 @@
 
 @interface GRPCOpSendMetadata : GRPCOperation
 
-- (instancetype)initWithMetadata:(id<GRPCRequestHeaders>)metadata
+- (instancetype)initWithMetadata:(GRPCRequestHeaders *)metadata
                          handler:(void(^)())handler NS_DESIGNATED_INITIALIZER;
 
 @end

src/objective-c/GRPCClient/private/GRPCWrappedCall.m

@@ -65,7 +65,7 @@
   return [self initWithMetadata:nil handler:nil];
 }
 
-- (instancetype)initWithMetadata:(id<GRPCRequestHeaders>)metadata handler:(void (^)())handler {
+- (instancetype)initWithMetadata:(GRPCRequestHeaders *)metadata handler:(void (^)())handler {
   if (self = [super init]) {
     _op.op = GRPC_OP_SEND_INITIAL_METADATA;
     _op.data.send_initial_metadata.count = metadata.count;