OpenTelemetry Tail Sampling: Full Fidelity on Errors, Sparse on Success

Head sampling:
 ingress decides keep/drop immediately
 → cheap, but drops failures that look fine at start

Tail sampling:
 all spans land in Collector briefly
 → policy: keep errors + p99 latency + 1% success
 → drop the rest

processors:
 tail_sampling:
 decision_wait: 10s
 num_traces: 100000
 expected_new_traces_per_sec: 20000
 policies:
 - name: errors
 type: status_code
 status_code: { status_codes: [ERROR] }
 - name: slow
 type: latency
 latency: { threshold_ms: 1500 }
 - name: checkout
 type: string_attribute
 string_attribute:
 key: http.route
 values: ["/v1/checkout", "/v1/pay"]
 enabled_regex_matching: false
 - name: probabilistic-rest
 type: probabilistic
 probabilistic: { sampling_percentage: 1 }

// App still records all spans locally; sampling decision is centralized
import { NodeSDK } from "@opentelemetry/sdk-node";
import { OTLPTraceExporter } from "@opentelemetry/exporter-trace-otlp-http";

const sdk = new NodeSDK({
 traceExporter: new OTLPTraceExporter({ url: process.env.OTEL_EXPORTER_OTLP_ENDPOINT }),
 // Prefer parent-based + Collector tail policies over aggressive SDK head drop
});