How does Micrometer work, and how do you design good metrics — tags, percentiles, and the mistakes that lie to you?

// Timer with bounded tags + histogram for percentile aggregation
Timer.builder("payment.charge")
     .tag("provider", provider.key())          // ~5 values ✅
     .tag("outcome", outcome)                  // success|declined|error ✅
     // .tag("orderId", orderId)               // ❌ cardinality bomb
     .publishPercentileHistogram()             // aggregatable across pods
     .register(registry)
     .record(() -> provider.charge(order));

// Gauge: reference must be strongly held — lambda over a weak ref silently NaNs
Gauge.builder("orders.queue.depth", queue, Queue::size).register(registry);