feat(metrics): chart host application Telemetry.Metrics definitions

[thiagoesteves]

What

Closes the main capability gap with Phoenix LiveDashboard: the Metrics page can now chart the host application's own Telemetry.Metrics definitions, not just the built-in VM/Phoenix series.

• config :observer_web, telemetry_metrics: [...] accepts a list of Telemetry.Metrics structs, or {module, function, args} for runtime-resolved definitions (e.g. reusing the list your other reporters consume).
• Definitions are appended to the built-in list consumed by ObserverWeb.Telemetry.Consumer, so custom series flow through the existing storage, retention, and Metric Hub (:local/:observer/:broadcast) modes unchanged - including history that survives app restarts in hub mode, which LiveDashboard cannot do.
• A new generic Custom chart component renders any metric key no dedicated component claims, labeling the series with its unit when available.
• Invalid entries or config shapes are dropped with a warning instead of breaking telemetry startup.
• Documented in the installation guide (Host application metrics (opt-in)) and the Observer.Web.Telemetry moduledoc.

Why

Part of the roadmap derived from comparing ObserverWeb against OTP observer, observer_cli and Phoenix LiveDashboard: rendering host-app telemetry was the main reason teams still run LiveDashboard alongside ObserverWeb.

Risk assessment

• Impact: opt-in; without the config, the metrics list and page rendering are unchanged (the generic chart only claims keys that previously rendered nothing).
• Blast radius: Observer.Web.Telemetry metrics list, Metrics page render loop, one new component; storage/consumer untouched.
• Regression risk: low - defensive config validation; suite green (403 tests, 96.0% coverage), credo/sobelow/dialyzer/format clean.
• Rollback plan: revert the commit; adopters just lose the extra charts.

Checklist

• mix test green (403 tests)
• mix coveralls 96.0% (threshold 95%)
• mix credo --strict, mix sobelow, mix dialyzer, mix format --check-formatted clean
• Small focused diff, no leftover debug output

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