feat(prometheus-client): add metric label about root on using Prome…

…theusClientLayer (#4907)

* refactor: revise the labels to be String

* factor out PrometheusClientMetrics with PrometheusClientMetricDefinitions

* refactor: remove the scheme label

* feat: add root tag to prometheus

* feat: add name into the labels

* refactor: make labels struct

* use Cow<'static, str>

* refactor: make the labels as struct