refactor: extract shared selftel + test helpers

[trilamsr]

Summary

Collapse four hand-rolled self-telemetry packages into one shared module/pkg/selftel + a module/pkg/testutil/selftel test helper. Each per-component selftel.go shrinks to a thin wrapper that pins scope name + instrument names + the package-local kind enum; the OTel counter / histogram / observable wiring, the degraded-state atomic machinery, and the init_errors_total fallback counter all live in one place.

Root cause

Four sibling ports landed independently (RFC-0013 PR-B1/PR-B2/PR-F.1) and each copy chose to re-implement the v0.1.x internal/selftelemetry shape locally rather than share. The result was ~860 LOC of verbatim duplication across components/exporters/{otlphttp,stdoutexporter}/selftel.go + components/receivers/pyspy/selftel.go + module/receiver/ncclfrreceiver/selftel.go, plus ~1600 LOC of test boilerplate (newTestMeterProvider, collectRM, findInstrument, scopeOf, attr-equality, and four near-identical failing*MP wrappers). This PR removes the duplication.

What stays per-component

• The kind string newtype + named consts (wire-format strings are the operator-facing contract).
• The instrumentationScope constant (each component's Go import path — OTel scope-name standard).
• The instrument name choices (e.g. otelcol.receiver.ncclfr.errors_total).
• A thin selfExporter / selfTelemetry interface that carries the package-local kind type at the seam (zero-cost cast to string).
• A package-local noopSelfExporter{} / noopSelfTelemetry{} that satisfies that local interface (a shared noop typed to selftel.Exporter / selftel.Receiver wouldn't fit through the kind-carrying interface).
• Test-side dumpNames/testID/testSettings/factory-shape tests stay where the factory lives.

What's now shared

• module/pkg/selftel.NewExporter(componentID, scope, callsTotalName, mp) + Exporter interface.
• module/pkg/selftel.NewReceiver(componentID, scope, names, mp) + Receiver interface + ReceiverInstrumentNames struct (named-field, not positional).
• module/pkg/selftel.RecordInitError(ctx, mp, scope, kind, componentID, reason) + ErrNilMeterProvider sentinel + ReasonInstrumentRegister const.
• module/pkg/testutil/selftel: NewTestMeterProvider, CollectRM, FindInstrument, ScopeOf, KVMatch, DumpNames, NewFailingMeterProvider(real, prefix).

Behavior contracts preserved

• Same metric names, same scope names, same label shapes (component_id, result, kind, etc.) — every pre-existing shape test passes unchanged.
• errors.Is(err, errNilMeterProvider) still holds in factories (sentinel aliased).
• recordInitError is still package-local; thin wrapper forwards to shared with the package's scope.
• The receiver state machine (lock-free CAS on degradedAt + accumulated nanoseconds + activityUnix gauge) is identical — moved verbatim into one place.
• Latency histogram bucket boundaries unchanged.

LOC delta

components/exporters/otlphttp/selftel.go            172 -> 117
components/exporters/otlphttp/selftel_test.go       417 -> 308
components/exporters/stdoutexporter/selftel.go      168 -> 110
components/exporters/stdoutexporter/selftel_test.go 404 -> 295
components/receivers/pyspy/selftel.go               249 -> 117
components/receivers/pyspy/selftel_test.go          389 -> 249
module/receiver/ncclfrreceiver/selftel.go           269 -> 130
module/receiver/ncclfrreceiver/selftel_test.go      388 -> 248
module/pkg/selftel/{selftel,exporter,receiver}.go   +384  (new)
module/pkg/selftel/selftel_test.go                  +244  (new)
module/pkg/testutil/selftel/selftel.go              +187  (new)

Net ~2456 -> ~2387 LOC, but ~860 LOC of verbatim duplication is gone — future changes to the receiver state machine / init-error counter / failing-meter wrapper touch one file instead of four.

Self-review pass (second commit)

Reviewing my own first commit caught dead exports: selftel.NoopExporter() and selftel.NoopReceiver() were unused by every caller (each per-component package owns its own noop typed to its package-local kind-carrying interface). Dropped both + their tests; added a comment at each removal site documenting why the export is intentionally absent so a future contributor doesn't re-add it under the same aspirational reasoning.

Test plan

• make check (fmt + tidy + golangci-lint + vet + mod-verify) — clean.
• go test ./components/... -count=1 — otlphttp / stdoutexporter / pyspy all green.
• cd module && go test ./... -count=1 — including new module/pkg/selftel shared tests + ncclfrreceiver + every other module package green.
• Per-package shape assertions (scope name, instrument name, attribute keys, datapoint shapes) pass — same tests that locked the v0.1.x contract still lock it.

NONE

Internal refactor — no operator-visible change. Same metric names, scope names, label shapes, and noop-fallback semantics.