fix(nimbus): correct featmon path and use $__all for Grafana metrics variable

[yashikakhurana]

Because

• The Grafana dashboard link on the Feature Health Dashboard was using var-metrics=All which Grafana does not recognise as "select all" — the correct value is $__all
• Features with monitoring configured in metric-hub were incorrectly showing "Grafana dashboard not available" because the featmon TOML path was pointing to a directory that does not exist in the Docker environment

This commit

• Updates FEATURE_MONITORING_URL to use var-metrics=$__all so the Grafana dashboard opens with all metrics selected
• Fixes METRIC_HUB_FEATMON_DESKTOP_PATH to point to outcomes/metric-hub-main/featmon/firefox_desktop.toml,

Fixes #16013

[jaredlockhart]

Awesome ty 🙏