int conversion fix
int conversion fix

got this message:
expr/parse.go:334:87: conversion from exprType (int) to string yields a
string of one rune, not a string of digits (did you mean fmt.Sprint(x)?)

raintank/metrictank,raintank/raintank-metric,raintank/raintank-metric,raintank/metrictank,raintank/metrictank,raintank/raintank-metric,raintank/metrictank