Increase the counter for a function with a certain cid.
string $function: The called function.
string $cid: The cache ID of the cache entry to increase the counter.
protected function increaseCounter($function, $cid) {
if (!isset($this->counter[$function][$cid])) {
$this->counter[$function][$cid] = 1;
}
else {
$this->counter[$function][$cid]++;
}
}