Update to use CustomMetricsOps

[dj707chen]

Adapt to change of http4s Add CustomMetricsOps:

To build a CustomMetricsOps[F, SL], replace method withCustomLabelsAndValues of Prometheus class with

def buildCustomMetricsOps[SL <: SizedSeq[String]](
      pCustomLabels: CustomLabels[SL]
  ): Resource[F, CustomMetricsOps[F, SL]]

Implement the original MetricsOps using EmptyCustomLabels.