chore: deprecate NewTCPx methods

Deprecate TCP APIs in favor of `github.com/mdlayher/netlink`.

Refer:
prometheus#622 (review).

Signed-off-by: Pranshu Srivastava <rexagod@gmail.com>

net_tcp.go

@@ -25,24 +25,32 @@ type (
 
 // NetTCP returns the IPv4 kernel/networking statistics for TCP datagrams
 // read from /proc/net/tcp.
+// Deprecated: Use github.com/mdlayher/netlink#Conn with syscall.AF_INET instead.
+// Refer: https://github.com/prometheus/node_exporter/pull/2322 for usage details.
 func (fs FS) NetTCP() (NetTCP, error) {
 	return newNetTCP(fs.proc.Path("net/tcp"))
 }
 
 // NetTCP6 returns the IPv6 kernel/networking statistics for TCP datagrams
 // read from /proc/net/tcp6.
+// Deprecated: Use github.com/mdlayher/netlink#Conn with syscall.AF_INET6 instead.
+// Refer: https://github.com/prometheus/node_exporter/pull/2322 for usage details.
 func (fs FS) NetTCP6() (NetTCP, error) {
 	return newNetTCP(fs.proc.Path("net/tcp6"))
 }
 
 // NetTCPSummary returns already computed statistics like the total queue lengths
 // for TCP datagrams read from /proc/net/tcp.
+// Deprecated: Use github.com/mdlayher/netlink#Conn with syscall.AF_INET instead.
+// Refer: https://github.com/prometheus/node_exporter/pull/2322 for usage details.
 func (fs FS) NetTCPSummary() (*NetTCPSummary, error) {
 	return newNetTCPSummary(fs.proc.Path("net/tcp"))
 }
 
 // NetTCP6Summary returns already computed statistics like the total queue lengths
 // for TCP datagrams read from /proc/net/tcp6.
+// Deprecated: Use github.com/mdlayher/netlink#Conn with syscall.AF_INET6 instead.
+// Refer: https://github.com/prometheus/node_exporter/pull/2322 for usage details.
 func (fs FS) NetTCP6Summary() (*NetTCPSummary, error) {
 	return newNetTCPSummary(fs.proc.Path("net/tcp6"))
 }