feat(micrometer): OpenTelemetry database semantic conventions for query observations

[zantvoort]

Observability backends key their database tooling on the OpenTelemetry database client semantic conventions over OTLP: latency panels, service-map database nodes, and query views recognize the standard db.* attributes. Storm's observations carried only storm.* key values, so its spans rendered as generic custom spans outside that tooling.

Fixes #231

What's included

• OtelDatabaseObservationConvention (storm-micrometer): extends the default convention with db.system.name and db.operation.name as low-cardinality key values and db.query.text as a high-cardinality value (parameter placeholders only, never values). The storm.* key values remain for custom dashboards, and db.statement is kept for backends that predate the stabilized conventions. fromJdbcUrl derives the database product from a JDBC URL using the conventions' well-known identifiers (mariadb, postgresql, h2database, oracle.db, microsoft.sql_server, ibm.db2; other_sql fallback).
• Spring Boot starters: storm.observations.semantic-conventions=otel activates the convention through the shared StormObservationAutoConfiguration, with the database product detected from the application's DataSource. A user-defined ObservationConvention bean still wins; an unknown property value fails startup with a descriptive error.
• Ktor plugin: an ObservationConvention<StormQueryObservationContext> registered in the dependency container overrides the automatic binding's naming and key values, mirroring the starters' convention bean.

The result is backend-specific database UX from vendor-neutral telemetry: the same spans light up whichever OTLP-capable backend the application points at — Elastic, Grafana, Datadog — with zero vendor-specific code in Storm or the application.

Verification

• Convention unit tests: standard attributes emitted alongside the storm.* key values; JDBC URL mapping for the well-known products.
• Java starter: end-to-end test asserting db.system.name=h2database on an observation recorded through the auto-configured template with the property set, plus the fail-fast case for unknown property values.
• Ktor: end-to-end test registering the convention in the dependency container and asserting the standard attributes on a live query observation.

Docs: Observability sections of spring-integration.md and ktor-integration.md, the yaml examples, and the changelog entry.