Governance: Extract a Generic expvar Package

[elliotxx]

What would you like to be added?

Extract a generic expvar package to replace the standard library's expvar package.

Why is this needed?

The current standard library's expvar package has an problem where it automatically includes 'cmdline' and 'memstats', and there is no way to remove them through configuration. The new generic expvar package should allow for customization of exposed variables through configuration, providing more flexibility and control over what metrics are exposed.