Micrometer — what is it? Metric types: counter, gauge, timer, distribution summary.

Micrometer is a vendor-neutral metrics facade — "SLF4J for metrics." You instrument your code against Micrometer's API once, and a pluggable MeterRegistry ships those meters to whichever backend you choose: Prometheus, Datadog, CloudWatch, New Relic, Graphite. Swap the registry dependency and the same instrumentation publishes somewhere else; your code never changes. Spring Boot auto-configures a MeterRegistry and pre-instruments JVM, HTTP, datasource, and cache metrics for you.

The four core meter types

• Counter — a monotonically increasing count. Use for events: requests served, orders placed, errors. You only ever increment().

  Counter orders = Counter.builder("orders.placed")
      .tag("channel", "web")
      .register(registry);
  orders.increment();

• Gauge — a value that goes up and down, sampled on scrape: queue depth, active sessions, cache size. You bind it to a function or object reference rather than setting it:

  Gauge.builder("queue.depth", queue, Queue::size).register(registry);

• Timer — measures both the count and the duration of short events (latency). It records total time, count, and max, and can emit percentiles/histograms.

  Timer timer = registry.timer("payment.duration");
  timer.record(() -> gateway.charge(order));   // times the lambda

• DistributionSummary — like a Timer but for non-time magnitudes: request payload size, batch row counts. Tracks count, total, max, and a distribution.

Tags (dimensions)

Every meter carries tags — key/value dimensions like method=GET, status=200, uri=/orders. They let you slice one metric many ways in Grafana. The rule: tags must be low-cardinality. Never tag with a user ID, order ID, or raw URL with path params — unbounded tag values explode the registry's memory and the backend's storage.

The Spring-idiomatic shortcut

@Timed on a controller or method auto-creates a Timer:

@Timed(value = "checkout.time", percentiles = {0.95, 0.99})
public Receipt checkout(Cart cart) { ... }

With the Prometheus registry on the classpath, all meters surface at /actuator/prometheus in scrape format.