imp: add flat fee for queries to VerifyMembership (#5927)

Co-authored-by: Damian Nolan <damiannolan@gmail.com>

modules/core/02-client/keeper/grpc_query.go

@@ -369,6 +369,7 @@ func (k Keeper) VerifyMembership(c context.Context, req *types.QueryVerifyMember
 	}
 
 	ctx := sdk.UnwrapSDKContext(c)
+
 	// cache the context to ensure clientState.VerifyMembership does not change state
 	cachedCtx, _ := ctx.CacheContext()
 
@@ -386,6 +387,13 @@ func (k Keeper) VerifyMembership(c context.Context, req *types.QueryVerifyMember
 		return nil, status.Error(codes.FailedPrecondition, errorsmod.Wrapf(types.ErrClientNotActive, "cannot verify membership using client (%s) with status %s", req.ClientId, clientStatus).Error())
 	}
 
+	// consume flat gas fee for proof verification queries.
+	// NOTE: consuming gas prior to method invocation also provides protection against recursive calls reaching stack overflow
+	ctx.GasMeter().ConsumeGas(
+		3*ctx.KVGasConfig().ReadCostPerByte*uint64(len(req.Proof)),
+		"verify membership query",
+	)
+
 	if err := clientState.VerifyMembership(cachedCtx, k.ClientStore(cachedCtx, req.ClientId), k.cdc, req.ProofHeight, req.TimeDelay, req.BlockDelay, req.Proof, req.MerklePath, req.Value); err != nil {
 		k.Logger(ctx).Debug("proof verification failed", "key", req.MerklePath, "error", err)
 		return &types.QueryVerifyMembershipResponse{