Add boot42-otel module: Spring Boot 4.2.0-RC1 OpenTelemetry conventions, shared OTLP settings and MDC keys vs 4.1.2

boot42-otel/README.md

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+# boot42-otel
+
+Companion module for the ankurm.com article *Spring Boot 4.2 OpenTelemetry: Semantic Conventions, Shared OTLP Settings and MDC Keys*.
+Everything runs against **Spring Boot 4.2.0-RC1** (a release candidate, **not GA**) and against **4.1.2** (GA) as the baseline. JDK 25.
+
+| Path | What it is |
+|---|---|
+| `pom.xml` / `pom-41.xml` | the same project on 4.2.0-RC1 and on 4.1.2 |
+| `src/main/.../OtelApplication` | one endpoint, `GET /hello/{name}`, that logs a line |
+| `src/test/.../OtlpSink` | a 60-line OTLP/HTTP receiver: records path, headers and payload strings of every export request |
+| `src/test/.../ConventionsTest` | meters and span names for each `management.observations.conventions` value, including the start-up failure RC1 gives for `open-telemetry` |
+| `src/test/.../ConventionSpellingsTest` | seven spellings of the conventions value, each started for real: which start and which meter they produce |
+| `src/test/.../SharedOtlpTest` | `management.opentelemetry.otlp.*`: paths, merged headers, gzip, and signal-specific overrides |
+| `src/test/.../MdcAndLongTaskTimerTest` | custom MDC keys, and the active-observation LongTaskTimer that is no longer on by default |
+| `src/test/.../PrometheusNamesTest` | the `http_server_*` names a dashboard query sees under each convention |
+| `output/` | every transcript quoted in the article; `./scripts/run-all.sh` regenerates them |
+
+Run: `./scripts/run-all.sh`. `ConventionsTest` pins the RC1 behaviour on purpose; when a later build changes it, that test fails and the article needs updating.
+
+There is deliberately no `docs/` folder: the explanations live in the article.