Escape label value

[moreamazingnick]

The label values can be like any strange text so this needs to be escaped.

php-prometheus/src/Serializer/PrometheusEncoder.php

Line 50 in 12bf879

$output[] = $key . '="' . $value . '"';

since prometheus does not know any non numeric metric values, the metric value would not need the escape function, but I don't want to break any compatibility.

php-prometheus/src/Serializer/PrometheusEncoder.php

Line 30 in 12bf879

$output[] = $labelledValue['name'] . $this->encodeLabels($labelledValue['labels']) . ' ' . $this->escapeValue($labelledValue['value']);

see:
prometheus/prometheus#2227

Best Regards
Nicolas