Fix observation example: drop hardcoded micrometer-tracing-bom import

observation/pom.xml imported micrometer-tracing-bom 1.5.0 ahead of
spring-boot-dependencies (which manages 1.7.1), forcing micrometer-core
down to 1.15.0. That version is missing the JvmMemoryMeterConventions
class, so service2 crashed at startup with a ClassNotFoundException
before service1/client could even be reached. spring-boot-dependencies
already manages micrometer-tracing consistently, so the extra BOM
import is removed instead of pinned to a matching version.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JvQJNT1UYzqUm5HQtkL6Nu