Frontend Observability Rum Instrumentation

Frontend Observability RUM Instrumentation

Purpose

Lab-only performance testing (Lighthouse, synthetic CI runs) systematically misses the device/network diversity of real users, and ad-hoc telemetry instrumentation routinely leaks PII into span attributes or blows up observability cost through unsampled high-cardinality export. This skill wires and reviews field RUM (web-vitals + OpenTelemetry Web) with explicit evidence labeling and privacy/cost guardrails baked in from the start.

When to use

Use this skill when the user asks to:

• instrument Core Web Vitals (LCP, INP, CLS) field measurement in a web app,
• set up or review OpenTelemetry Web browser tracing (document load, fetch/XHR spans),
• review an existing RUM/telemetry pipeline for PII exposure or sampling/cost issues,
• interpret or report on Core Web Vitals data and needs a lab-vs-field distinction made explicit,
• set or validate a performance budget against field p75 data.

When NOT to use

• Diagnosing why a specific LCP/INP/CLS number is high (phase decomposition of an existing regression) — hand off to core-web-vitals-triage; this skill instruments the pipeline that produces the data, it does not decompose an already-captured regression.
• Bundle size, code-splitting, or caching remediation once a cause is known — hand off to bundle-budget-code-splitting-review or service-worker-cache-strategy-review.
• Server-side or backend-service tracing with no browser-origin span — this skill is scoped to browser-side RUM and OpenTelemetry Web only.

Context7 Documentation Protocol

web-vitals and OpenTelemetry Web packages ship breaking API changes across major versions (attribution-build option shapes, sampler class names, exporter constructor options), and memorized snippets go stale. Before writing or reviewing instrumentation code:

1. Call ToolSearch with query "context7" (or "select:mcp__Context7__resolve-library-id,mcp__Context7__query-docs") to load the Context7 tools if not already loaded this session.
2. Call mcp__Context7__resolve-library-id for web-vitals and/or opentelemetry-js before making any claim about their current API shape. Verified IDs as of this skill's updated date: /googlechrome/web-vitals and /open-telemetry/opentelemetry-js.
3. Call mcp__Context7__query-docs for the specific mechanism in scope — e.g. "attribution build generateTarget option", "INPAttributionReportOpts durationThreshold", "WebTracerProvider spanProcessors config", "TraceIdRatioBasedSampler" — before ruling on it. Do this per review; do not reuse a prior session's memory of these APIs.
4. Known facts verified via Context7 as of this skill's updated date: the attribution build (web-vitals/attribution) accepts AttributionReportOpts (reportAllChanges, generateTarget) on onCLS/onFCP/onLCP/onTTFB, and INPAttributionReportOpts additionally exposes durationThreshold (default 40) and includeProcessedEventEntries (default true) on onINP. WebTracerProvider (from @opentelemetry/sdk-trace-web) takes a spanProcessors array in its constructor — do not tell a user to call a separate addSpanProcessor method as the primary pattern without checking the installed SDK version. TraceIdRatioBasedSampler takes a single ratio argument (0–1) and is normally wrapped in ParentBasedSampler({ root: ... }) so that downstream/parent sampling decisions are respected.
5. If Context7 is unavailable or returns no relevant match, fall back to official_docs / references/*.md and mark the claim documentation-based (Context7 unavailable) rather than presenting it as freshly verified.
6. Never invent a web-vitals metric field, attribution property, OpenTelemetry exporter option, or sampler class that no queried source confirms.

Lean operating rules

• Always label every performance number as lab evidence (Lighthouse/CI/synthetic) or field evidence (RUM, real users — state the percentile; p75 is the CWV standard) — never let the two blur into an unlabeled "the site is fast" claim.
• Default to the standard web-vitals build (onLCP/onINP/onCLS) for production reporting; only use the /attribution build's richer diagnostic payload when actively debugging a specific regression, and only send that extra payload to a dev/debug destination, not blanket production export.
• Do not enable reportAllChanges: true in production by default — it multiplies event volume for a debugging-only benefit.
• Before adding any custom span/metric attribute, run it through a PII check: no raw URLs with query strings, no user IDs, no free-text form input, no precise geolocation, unless explicitly scrubbed and justified.
• Size the OpenTelemetry sampling rate (e.g. TraceIdRatioBasedSampler wrapped in ParentBasedSampler) against the stated/estimated traffic volume and the backend's cost/cardinality limits; never recommend 100% unsampled export for a high-traffic app without an explicit cost review.
• Use OpenTelemetry Semantic Conventions attribute names instead of inventing ad-hoc span/attribute naming, so telemetry stays queryable/comparable across services.
• Treat any recommended performance budget threshold as grounded in web.dev's documented "good" ranges (LCP ≤2.5s, INP ≤200ms, CLS ≤0.1 at p75) unless the org has an explicitly stricter documented SLO.
• This skill performs static review and instrumentation-code authoring only; it does not deploy telemetry configuration to production or flip sampling/export settings on a live collector without explicit human sign-off logged outside this skill.

References

Load these only when needed:

• web-vitals attribution build wiring — use when writing or reviewing the actual onLCP/onINP/onCLS instrumentation call, choosing between standard and attribution builds, or setting generateTarget/durationThreshold/reportAllChanges.
• OpenTelemetry Web tracing wiring — use only when wiring or reviewing WebTracerProvider, DocumentLoadInstrumentation, FetchInstrumentation, exporter, or sampler configuration.
• Sampling, cardinality, and PII controls — use when sizing sampling rates against traffic/cost, naming attributes via Semantic Conventions, or auditing an existing pipeline for PII leakage.

Response minimum

Return, at minimum:

• the metric/trace component in scope and whether guidance concerns lab or field measurement,
• evidence level for any performance claim (lab-evidence, field-evidence with percentile, or documentation-based),
• instrumentation code or review findings with exact API options used,
• PII-in-telemetry check result for every attribute touched,
• sampling-rate rationale tied to stated traffic volume and cost constraints.