upstream: stats scope for dual filters

[alyssawilk]

https://github.com/envoyproxy/envoy/blob/main/source/docs/upstream_filters.md calls out how to convert filters to dual filters and notes that init_manager and stats must come from DualInfo

This pr actually adds stats to DualInfo for use in #23071 and updates docs for filter conversion

Risk Level: low
Testing: updated unit test
Docs Changes: n/a
Release Notes: n/a

[alyssawilk]

I'm also going to update the docs in this PR, so the other is a clean example and doesn't include spurious files.

[wbpcode]

Please resolve the conflict in the docs. Thanks.

[wbpcode]

And a minor spelling error in doc:

Either the filter does not access streamInfo in a non-cost way

non-cost -> non-const

[wbpcode]

And could you check following snippet of doc:

  * Add ``UpstreamMyFilterFactory = MyFilterFactory;`` and
    ``DECLARE_FACTORY(UpstreamMyFilterFactory`` in your config.h file and
    ``REGISTER_FACTORY(UpstreamMyFilterFactory,
                     Server::Configuration::UpstreamHttpFilterConfigFactory){"envoy.my_filter"};`` to
    your config.cc file.

Seems it lost something like using or );.

And if DECLARE_FACTORY is necessary? I didn't find it in the #23071.

[wbpcode]

LGTM. Thanks.