refactor: move bytecounting conn in bytecounter pkg (#392)

* refactor: move bytecounting conn in bytecounter pkg

This enables other pieces of code to request bytecounting without
depending on netx or on the perverse using-the-context-to-configure-
byte-counting mechanism.

Also occurred when working on https://github.com/ooni/probe/issues/1687

* fix: add missing docs

internal/bytecounter/conn.go

@@ -0,0 +1,26 @@
+package bytecounter
+
+import "net"
+
+// Conn wraps a network connection and counts bytes.
+type Conn struct {
+	// net.Conn is the underlying net.Conn.
+	net.Conn
+
+	// Counter is the byte counter.
+	Counter *Counter
+}
+
+// Read implements net.Conn.Read.
+func (c *Conn) Read(p []byte) (int, error) {
+	count, err := c.Conn.Read(p)
+	c.Counter.CountBytesReceived(count)
+	return count, err
+}
+
+// Write implements net.Conn.Write.
+func (c *Conn) Write(p []byte) (int, error) {
+	count, err := c.Conn.Write(p)
+	c.Counter.CountBytesSent(count)
+	return count, err
+}