Scrape jmx-metrics weaver models into the registry

[jaydeluca]

The JMX instrumentation in the java agent now has weaver models with information about the telemetry emitted:

https://github.com/open-telemetry/opentelemetry-java-instrumentation/tree/main/instrumentation/jmx-metrics/model

We should update the https://github.com/open-telemetry/opentelemetry-ecosystem-explorer/tree/main/ecosystem-automation/java-instrumentation-watcher to scrape these, and then we can figure out how we want to use/display it.

The weaver files are newish, so we need to make sure the watcher accounts for when they don't exist and just ignores them and doesn't fail in the event we run a backfill.

We can extend InstrumentationSync to fetch the instrumentation/jmx-metrics/model/*.yaml files at the same ref it already uses for instrumentation-list.yaml, and persist them content-addressed.

Model files change rarely and are identical across most releases, so store them once in a
shared directory at the ecosystem root, with a small per-version index pointing into it:

ecosystem-registry/java/javaagent/
├── jmx/                                      # shared, content-addressed model store
│   ├── activemq-6c55bb962678.yaml
│   ├── camel-e74b2ed35478.yaml
│   ├── jetty-a3bae406cfcf.yaml
│   ├── jvm-45b20f5f3f0e.yaml
│   ├── manifest-977075160b7e.yaml
│   ├── tomcat-5846db79cc39.yaml
│   └── wildfly-6d2c8fd10301.yaml
├── v2.29.0/
│   └── instrumentation.yaml                  # no jmx-models.yaml — models don't exist yet
└── v2.30.0/
    ├── instrumentation.yaml
    ├── jmx-models.yaml                       # index for this version
    └── library_readmes/

v2.30.0/jmx-models.yaml:

models:
  activemq: activemq-6c55bb962678.yaml
  camel: camel-e74b2ed35478.yaml
  jetty: jetty-a3bae406cfcf.yaml
  jvm: jvm-45b20f5f3f0e.yaml
  tomcat: tomcat-5846db79cc39.yaml
  wildfly: wildfly-6d2c8fd10301.yaml
manifest: manifest-977075160b7e.yaml

Notes on this shape:

• The keys are JMX target systems, not library names. Each model basename matches a value accepted by the otel.jmx.target.system config option, which corresponds to a rule file in instrumentation/jmx-metrics/library/src/main/resources/jmx/rules/. There is exactly one
  instrumentation here (jmx-metrics); the models describe its target systems.
• manifest.yaml is not a model — it is the weaver registry manifest, and it carries the pinned semconv version (semantic-conventions@v1.43.0) that the next phase of this project will need to resolve attribute refs. Store it, but under a separate top-level key so it is never iterated as a target system.
• The shared-store layout matches where Deduplicate readmes in registry #883 is taking library_readmes/. Please land it this way rather than nesting per-version, so the two converge. If Deduplicate readmes in registry #883 lands first, follow whatever helper it introduces.

[jaydeluca]

@pratik50 if you are interested

[pratik50]

/assign