consumer: allow disabling fetch sessions with a config opt

Sometimes users may not want these.

pkg/kgo/config.go

@@ -135,6 +135,7 @@ type cfg struct {
 	rack           string
 
 	maxConcurrentFetches int
+	disableFetchSessions bool
 
 	topics     map[string]*regexp.Regexp   // topics to consume; if regex is true, values are compiled regular expressions
 	partitions map[string]map[int32]Offset // partitions to directly consume from
@@ -1193,6 +1194,35 @@ func ConsumeRegex() ConsumerOpt {
 	return consumerOpt{func(cfg *cfg) { cfg.regex = true }}
 }
 
+// DisableFetchSessions sets the client to not use fetch sessions (Kafka 1.0+).
+//
+// A "fetch session" is is a way to reduce bandwidth for fetch requests &
+// responses, and to potentially reduce the amount of work that brokers have to
+// do to handle fetch requests. A fetch session opts in to the broker tracking
+// some state of what the client is interested in. For example, say that you
+// are interested in thousands of topics, and most of these topics are
+// receiving data only rarely. A fetch session allows the client to register
+// that it is interested in those thousands of topics on the first request.  On
+// future requests, if the offsets for these topics have not changed, those
+// topics will be elided from the request. The broker knows to reply with the
+// extra topics if any new data is available, otherwise the topics are also
+// elided from the response. This massively reduces the amount of information
+// that needs to be included in requests or responses.
+//
+// Using fetch sessions means more state is stored on brokers. Maintaining this
+// state eats some memory. If you have thousands of consumers, you may not want
+// fetch sessions to be used for everything. Brokers intelligently handle this
+// by not creating sessions if they are at their configured limit, but you may
+// consider disabling sessions if they are generally not useful to you. Brokers
+// have metrics for the number of fetch sessions active, so you can monitor
+// that to determine whether enabling or disabling sessions is beneficial or
+// not.
+//
+// For more details on fetch sessions, see KIP-227.
+func DisableFetchSessions() ConsumerOpt {
+	return consumerOpt{func(cfg *cfg) { cfg.disableFetchSessions = true }}
+}
+
 //////////////////////////////////
 // CONSUMER GROUP CONFIGURATION //
 //////////////////////////////////

pkg/kgo/source.go

@@ -49,6 +49,9 @@ func (cl *Client) newSource(nodeID int32) *source {
 		nodeID: nodeID,
 		sem:    make(chan struct{}),
 	}
+	if cl.cfg.disableFetchSessions {
+		s.session.kill()
+	}
 	close(s.sem)
 	return s
 }