[Feature]: Data-driven generic template for Metricbeat modules (replace beats_metricbeat_modules)

Mirror of #510 for Metricbeat. Today beats_metricbeat_modules is a flat list of module names; the role only runs metricbeat modules enable, so a module's config can't be set — hosts, xpack.enabled, credentials, ssl (i.e. targeting).

Replace the flat beats_metricbeat_modules name-list with a data-driven variable (a list of modules with their config), rendered inline into metricbeat.yml (metricbeat.modules:) by the shared template/macro. This one list is the declarative desired state (in the list = enabled+configured; removed = gone; no separate disable list, no enabled: false). Render inline, not modules.d/*.yml (inline re-rendered wholesale → clean removal; modules.d orphans files and its settings take precedence). Drop metricbeat modules enable; leave modules.d/ untouched. Re-adapt the setup task here too (supersedes #508's current-architecture fix): run metricbeat setup on this template's change (register/is changed, run_once); inline means no --modules needed.

This also covers Stack Monitoring via the xpack.enabled field:

• xpack.enabled: false (current) → metrics land in the normal metricbeat-* indices; the user builds their own dashboards.
• xpack.enabled: true → data is written to .monitoring-*, usable by the ready-made Kibana "Stack Monitoring" app.

In a full stack the targeting defaults to the known hosts/CA/monitoring user. On 8.x there is no Stack Monitoring out of the box (legacy xpack.monitoring push is 7-only).

Docs: shared roles/beats/docs/beats-modules.md (common model + a Metricbeat section incl. an xpack.enabled example), linked from the beats_metricbeat_modules description; README table shows only a short default + the pointer.

Breaking: beats_metricbeat_modules changes from a name list to a data structure. Shares the macro + docs page with #510 and the Auditbeat modules issue. Related: #77 (Elastic Agent); legacy monitoring removed with #446.

Depends on: #393 (shared plugin-as-data macro).

Start with the shared plugin-as-data macro from dependency #393 and the Metricbeat role's metricbeat.yml template and setup task. Read roles/beats/docs/beats-modules.md for the common model and Metricbeat section. Done means the data-driven list renders inline, setup runs when that template changes, modules.d remains untouched, and the README and variable description point to the documentation.