This article helps explain the different data types used in custom metrics.
Counters - The Metrics whose value keeps on increasing is assumed as of Counter type. For example, Server Uptime, as the value of server uptime never decreases, but keeps on incrementing.
Gauges - The Metrics whose values may fluctuate over time i.e. increment or decrement, is assumed to be of gauge type. For example, Network throughput of a network interface.