getMetrics = function(
    symbol: string,
    font: string,
    mode: Mode,
): CharacterMetrics {
    const replace = symbols.math[symbol] && symbols.math[symbol].replace;
    const metrics =
        getCharacterMetrics(replace || symbol, font, mode);
    if (!metrics) {
        throw new Error(`Unsupported symbol ${symbol} and font size ${font}.`);
    }
    return metrics;
}
Get the metrics for a given symbol and font, after transformation (i.e.
after following replacement from symbols.js)