Track ActiveRequest count for transport connection

[chopraanmol1]

While the go-elasticsearch library allows user to have custom connection pool selector, the currently available details to selector does not allow user to switch to a more mature client side load balancing selector. As part of this issue, will suggest adding ActiveRequest count on following struct

• github.com/elastic/go-elasticsearch/v7/estransport.Connection
• github.com/elastic/elastic-transport-go/v8/elastictransport.Connection

which will allow user to implement load balancing algorithm like envoy proxy equivalent LEAST_REQUEST(either power of 2 choice, or power of N choice) or HAProxy equivalent leastconn on client side. This change will make it much easier for user to switch to more appropriate load balancing selector when round robin is not sufficient for the usecase. If needed LEAST_REQUEST selector support could be added in the client library itself.

Can help with raising relevant Pull requests for same. Also as elasticsearch 7.17 has not reached EOL, will request having this change implemented for both v7 & v8.