receiver/kafkareceiver: correct balancer docs (open-telemetry#43894)

#### Description

Correct the docs to state that the default balancer is
"cooperative-sticky" when using franz-go (now the default), and "range"
only when using Sarama.

#### Link to tracking issue

Fixes open-telemetry#43892

#### Testing

N/A

#### Documentation

Yes

Author: axw

.chloggen/kafkareceiver-fix-docs.yaml

@@ -0,0 +1,27 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: bug_fix
+
+# The name of the component, or a single word describing the area of concern, (e.g. receiver/filelog)
+component: receiver/kafka
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Corrected the documentation for the Kafka receiver to accurately the supported/default group balancer strategies.
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [43892]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:
+
+# If your change doesn't affect end users or the exported elements of any package,
+# you should instead start your pull request title with [chore] or use the "Skip Changelog" label.
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: [user]

pkg/kafka/configkafka/config.go

@@ -124,8 +124,10 @@ type ConsumerConfig struct {
 	MaxPartitionFetchSize int32 `mapstructure:"max_partition_fetch_size"`
 
 	// RebalanceStrategy specifies the strategy to use for partition assignment.
-	// Possible values are "range", "roundrobin", and "sticky".
-	// Defaults to "range".
+	// Possible values are "range", "roundrobin", and "sticky", and
+	// "cooperative-sticky" (franz-go only).
+	//
+	// Defaults to "cooperative-sticky" for franz-go, "range" for Sarama.
 	GroupRebalanceStrategy string `mapstructure:"group_rebalance_strategy,omitempty"`
 
 	// GroupInstanceID specifies the ID of the consumer

receiver/kafkareceiver/README.md

@@ -64,10 +64,11 @@ The following settings can be optionally configured:
 - `initial_offset` (default = latest): The initial offset to use if no offset was previously committed. Must be `latest` or `earliest`.
 - `session_timeout` (default = `10s`): The request timeout for detecting client failures when using Kafka’s group management facilities.
 - `heartbeat_interval` (default = `3s`): The expected time between heartbeats to the consumer coordinator when using Kafka’s group management facilities.
-- `group_rebalance_strategy` (default = `range`): This strategy is used to assign partitions to consumers within a consumer group. This setting determines how Kafka distributes topic partitions among the consumers in the group during rebalances. Supported strategies are:
+- `group_rebalance_strategy` (default = `cooperative-sticky` (franz-go), 'range' (Sarama)): This strategy is used to assign partitions to consumers within a consumer group. This setting determines how Kafka distributes topic partitions among the consumers in the group during rebalances. Supported strategies are:
   - `range`: This strategy assigns partitions to consumers based on a range. It aims to distribute partitions evenly across consumers, but it can lead to uneven distribution if the number of partitions is not a multiple of the number of consumers. For more information, refer to the Kafka RangeAssignor documentation, see [RangeAssignor](https://kafka.apache.org/31/javadoc/org/apache/kafka/clients/consumer/RangeAssignor.html).
   - `roundrobin`: This strategy assigns partitions to consumers in a round-robin fashion. It ensures a more even distribution of partitions across consumers, especially when the number of partitions is not a multiple of the number of consumers. For more information, refer to the Kafka RoundRobinAssignor documentation, see [RoundRobinAssignor](https://kafka.apache.org/31/javadoc/org/apache/kafka/clients/consumer/RoundRobinAssignor.html).
   - `sticky`: This strategy aims to maintain the same partition assignments during rebalances as much as possible. It minimizes the number of partition movements, which can be beneficial for stateful consumers. For more information, refer to the Kafka StickyAssignor documentation, see [StickyAssignor](https://kafka.apache.org/31/javadoc/org/apache/kafka/clients/consumer/StickyAssignor.html).
+  - `cooperative-sticky`: This strategy is similar to `sticky`, but it supports cooperative rebalancing. It allows consumers to incrementally adjust their partition assignments without requiring a full rebalance, which can reduce downtime during rebalances. For more information, refer to the Kafka CooperativeStickyAssignor documentation, see [CooperativeStickyAssignor](https://kafka.apache.org/31/javadoc/org/apache/kafka/clients/consumer/CooperativeStickyAssignor.html).
 - `group_instance_id`: A unique identifier for the consumer instance within a consumer group.
   - If set to a non-empty string, the consumer is treated as a static member of the group. This means that the consumer will maintain its partition assignments across restarts and rebalances, as long as it rejoins the group with the same `group_instance_id`.
   - If set to an empty string (or not set), the consumer is treated as a dynamic member. In this case, the consumer's partition assignments may change during rebalances.