I think this construction means that adding the same dimension with different values will silently do nothing the second time, which is maybe a little weird:

$metric->addDimension('a', 1);
$metric->addDimension('a', 2); // Does nothing.

Consider:

$this->dimensions[$name] = $value;

...to make a second call override, or an array_key_exists() test plus throw ... to make a second call an error.