refactor(appkit): let plugins contribute otel span processors

[MarioCadenas]

What

Splits TelemetryManager into two phases so a single global OpenTelemetry tracer provider can carry both the OTLP trace exporter and span processors contributed by plugins — instead of each SDK (AppKit's and, e.g., MLflow's) racing to register the process-global provider, where the second registration is silently dropped.

• initialize() — registers the meter and logger providers eagerly (unchanged timing, at app bootstrap). This is deliberate: OTel's metrics API has no lazy proxy, so a counter/histogram bound against the NoOp meter (as connectors and the cache do in their constructors) stays NoOp for the process lifetime. It no longer registers a tracer provider.
• registerSpanProcessor(sp) — new; lets a plugin contribute a span processor during setup(). Ignored with a warning after start(), since a started provider's processors are immutable in OTel JS 2.x.
• start() — new; called by core after all plugin setup() completes. Builds the global NodeTracerProvider with the OTLP processor (when configured) plus every contributed processor, and no-ops when nothing needs tracing. NodeTracerProvider.register() reinstates the async-hooks context manager and W3C propagators that NodeSDK.start() used to provide, so span nesting across awaits is preserved.
• shutdown() now flushes all three providers.

Also swaps the @opentelemetry/sdk-node dependency for @opentelemetry/sdk-trace-node, since the tracer provider is now built directly rather than via NodeSDK.

Why

When both an OTLP endpoint and a plugin tracer (MLflow) are active, they each build a NodeSDK and call .start(), which registers the global tracer provider. OTel's registerGlobal uses allowOverride=false, so the second registration is rejected — and because AppKit initializes first, MLflow's spans silently route to a provider that has no MLflow processor. OTel 2.x removed addSpanProcessor(), so the only way to have both coexist is one provider built after every processor is known — which requires deferring the tracer provider past plugin setup.

Deferring is safe for traces (verified against the OTel source): ProxyTracer rebinds tracers obtained before registration, and no span is emitted during setup(). It is not safe for metrics, hence the eager meter/logger split above.

Scope

This is the enabling infrastructure only. No plugin calls registerSpanProcessor() yet — wiring the MLflow plugin to contribute its processor is a follow-up, gated on mlflow-tracing exposing its MlflowSpanProcessor (it is not exported in 0.1.3).

Verification

Re-run after merging main up to date (past the Biome → oxlint/oxfmt migration in #538):

• pnpm --filter=@databricks/appkit typecheck — clean
• Telemetry suite — 44 pass, including new two-phase tests (contributed-only-no-OTLP routing, idempotent start(), post-start() register ignored, no-op when unconfigured, and a guard that a metric instrument obtained before start() still records)
• knip — clean; oxlint + oxfmt — clean

---

This pull request and its description were written by Isaac.

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📦 Bundle size report

Compared against bundle-size-baseline.json (main).

@databricks/appkit

npm tarball (packed): 912 KB (+25 KB) — gzipped download (dist + bin; excludes release-only docs/NOTICE).

dist	raw	gzip
JS (runtime)	940 KB (+34 KB)	327 KB (+9.6 KB)
Type declarations	345 KB (+1.3 KB)	120 KB (+512 B)
Source maps	1.8 MB (+64 KB)	612 KB (+16 KB)
Other	11 KB	3.7 KB
Total	3.1 MB (+99 KB)	1.0 MB (+26 KB)

Per-entry composition (own code — deps external (as shipped))

Entry	Initial (gz)	Lazy (gz)	Total (gz)	node_modules (min)	Own code (min)
.	94 KB (+5.2 KB)	2.5 KB	96 KB (+5.2 KB)	external	307 KB (+19 KB)
./beta	56 KB (+5.3 KB)	457 B	56 KB (+5.3 KB)	external	167 KB (+20 KB)
./testing	17 KB (+327 B)	0 B	17 KB (+327 B)	external	51 KB (+1.3 KB)
./tsdown	520 B	0 B	520 B	external	813 B
./type-generator	21 KB	0 B	21 KB	external	61 KB

Chunks:

Entry	Chunk	Load	Size (gz)
.	index.js	initial	90 KB
.	utils.js	initial	4.0 KB
.	remote-tunnel-manager.js	lazy	2.5 KB
./beta	beta.js	initial	40 KB
./beta	stream-manager.js	initial	5.8 KB
./beta	wide-event-emitter.js	initial	3.2 KB
./beta	databricks.js	initial	3.0 KB
./beta	configuration.js	initial	2.1 KB
./beta	service-context.js	initial	1.3 KB
./beta	client.js	initial	434 B
./beta	client-options.js	initial	220 B
./beta	supervisor-api.js	lazy	192 B
./beta	databricks.js	lazy	142 B
./beta	index.js	lazy	123 B
./testing	index.js	initial	17 KB
./tsdown	index.js	initial	520 B
./type-generator	index.js	initial	21 KB

@databricks/appkit-ui

npm tarball (packed): 348 KB (+9 B) — gzipped download (dist + bin; excludes release-only docs/NOTICE).

dist	raw	gzip
JS (runtime)	394 KB	132 KB
Type declarations	228 KB (+32 B)	83 KB (+9 B)
Source maps	764 KB	252 KB
CSS	16 KB	3.2 KB
Total	1.4 MB (+32 B)	471 KB (+9 B)

Per-entry composition (consumer bundle — deps bundled, peerDeps external)

Entry	Initial (gz)	Lazy (gz)	Total (gz)	node_modules (min)	Own code (min)
./js	5.3 KB	49 KB	55 KB	208 KB	14 KB
./js/beta	20 B	0 B	20 B	0 B	0 B
./react	432 KB	49 KB	481 KB	1.3 MB	177 KB
./react/beta	1.0 KB	0 B	1.0 KB	0 B	1.9 KB

Chunks:

Entry	Chunk	Load	Size (gz)
./js	index.js	initial	5.2 KB
./js	chunk	initial	120 B
./js	apache-arrow	lazy	49 KB
./js/beta	beta.js	initial	20 B
./react	index.js	initial	430 KB
./react	tslib	initial	2.1 KB
./react	apache-arrow	lazy	49 KB
./react/beta	beta.js	initial	1.0 KB

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