[dnsdist] backend drops metrics for TCP

[Christoph]

Hello!

when playing around with things to reduce the drop rate I noticed
that TCP based backends always have 0 drops in showServers() output and 
these metrics:
dnsdist_server_drops
dnsdist_downstream_timeouts

Is that always the case and that counter has no meaning for TCP based 
backends or can this counter be non-zero for TCP backends as well?

dnsdist's CPU usage doubled after switching to TCP via tcpOnly=true
and the DNS timeout rate as measured by the application generating the 
queries running on the same host as dnsdist actually increased after 
switching dnsdist to use TCP instead of UDP. So switching to TCP 
eliminated the drops problem when measured by dnsdist but it made things 
worse for the application.

All of these values are also at 0:

dnsdist_server_tcpdiedsendingquery{address="127.0.0.1:54"} 0
dnsdist_server_tcpdiedreadingresponse{address="127.0.0.1:54"} 0
dnsdist_server_tcpgaveup{address="127.0.0.1:54"} 0
dnsdist_server_tcpreadtimeouts{address="127.0.0.1:54"} 0
dnsdist_server_tcpwritetimeouts{address="127.0.0.1:54"} 0
dnsdist_server_tcpconnecttimeouts{address="127.0.0.1:54"} 0

dnsdist_server_latency and
dnsdist_server_tcplatency
are on the same level after switching to TCP for the specific backend.

Since sockets=NUM in newServer() is only for UDP and
dnsdist_server_tcpcurrentconnections{address="127.0.0.1:54"} 10
suggests it uses only 10 TCP sockets. How can this be configured?
sockets was set to 32, so this implicit change when sitching from UDP to 
TCP might also have an effect here.

best regards,
Christoph