Monitor / view current TCP connection routing for long lived connections

Note: This might be related to https://github.com/envoyproxy/envoy/issues/3818 .

Using envoyproxy/envoy:v1.9.0 .

We have a requirement / enhancement request to see what TCP connections are currently active. What would quickly help us out, is to add an option to the access logger to log at (established) connect time, next to the disconnect time.

To be able to query the admin API for this would be great as well.

More details:

We use Envoy for long lived TCP connections, with two-way trust and SNI. Via SNI, a dedicated cluster is selected, via filter_chain_match. TCP connections are long lived (can be days). Currently, the access logger only logs at disconnect (and for stats like bytes_recieved this makes sense).