Issue Template

Title: *Http2: Need connection policy to allow and control multiple upstream connections. *

Description:

This is related to #7217.

Current default upstream connection scheme doesnt allow you to set a maximum number of streams allowed per connection. This causes upstream load balancing issues as some connections could carry more streams. For example: when upstream is a multiprocess server with shared socket, only few workers receive all the streams.

A connection policy that gives configurable parameters to manage maximum number of streams per upstream connections is needed to solve this problem.

Some implementation approaches:

• Use current circuit breaking codepath and use the onPoolFailure method to add a case for maxStreamsOverflow.

• Add another callback in Http::ConnectionPool::Callbacks to handle this case.

Essentially we need to :

• Enqueue the stream if current client stream count exceeds the configured maximum
• Create a new connection
• Process the enqueued stream/request in the context of the new connection.