[BUG] Broken plugins metrics support

[mrgian]

libs 0.17.0 introduced the support for plugin-provided metrics in the plugin API. However, this new feature is currently broken and should not be used.

Even if present in the plugin API (https://github.com/falcosecurity/libs/blob/master/userspace/plugin/plugin_api.h), the symbol get_metrics should not be implemented by plugins.

The issue lies in the symbol get_metrics (https://github.com/falcosecurity/libs/blob/master/userspace/plugin/plugin_api.h#L979) not being registered when the plugin is loaded. For this reason sinsp will ignore any metric provided by plugins using this feature.