Counter is a cumulative metric that represents a single numerical value that only ever goes up.

Gauge is a metric that represents a single numerical value that can arbitrarily go up and down.

Histogram samples observations and counts them in configurable buckets.

Summary samples observations (usually things like request durations) and provides configurable quantiles.

Metrics represents a standard metrics interface.

MetricType represents the possible types of metrics (Counter, Gauge, Summary, Histogram).