Grafana variable in square brackets


I need to use variable in square brackets followed by string and I can’t find a way to do this.
I use the rate function in this way rate(<metric_name>[60s]) where 60 is my variable and I need to concatenate “s” after it.
I tried [[[varname]]s] but this doesn’t work.It seems like the syntax [[varname]] doesn’t work if the previous symbol is “[”.
I can’t use the syntax $varname as well, because I need to concatenate string “s” after the variable.
Please advise how to resolve this case.