[receiver/azureeventhubreceiver] use $Default as default consumer group (#43051)

<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->
#### Description
The old SDK sets `$Default` as the consumer group when no consumer
groups are set. This is a fix to the new SDK implementation to do the
same. This would cause issues when users attempted to use the new SDK
and didn't have the consumer group set.

<!-- Issue number (e.g. #1234) or full URL to issue, if applicable. -->
#### Link to tracking issue
Fixes #43049 

<!--Describe what testing was performed and which tests were added.-->
#### Testing
Added tests for the function for getting the consumer group


<!--Please delete paragraphs that you did not use before submitting.-->

Author: dyl10s

.chloggen/dylan_azure-event-hubs-default-consumer-group.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. filelogreceiver)
+component: azureeventhubreceiver
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Use `$Default` as the default consumer group with the new azeventhubs SDK
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [43049]
+
+# (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]

receiver/azureeventhubreceiver/eventhubhandler_azeventhub.go

@@ -23,11 +23,20 @@ type azPartitionClient interface {
 	ReceiveEvents(ctx context.Context, maxBatchSize int, options *azeventhubs.ReceiveEventsOptions) ([]*azeventhubs.ReceivedEventData, error)
 }
 
+func getConsumerGroup(config *Config) string {
+	if config.ConsumerGroup == "" {
+		return "$Default"
+	}
+	return config.ConsumerGroup
+}
+
 func newAzeventhubWrapper(h *eventhubHandler) (*hubWrapperAzeventhubImpl, error) {
+	consumerGroup := getConsumerGroup(h.config)
+
 	hub, newHubErr := azeventhubs.NewConsumerClientFromConnectionString(
 		h.config.Connection,
 		"",
-		h.config.ConsumerGroup,
+		consumerGroup,
 		&azeventhubs.ConsumerClientOptions{},
 	)
 
@@ -124,7 +133,6 @@ func (h *hubWrapperAzeventhubImpl) Receive(ctx context.Context, partitionID stri
 				}
 			}
 		}
-
 		pc, err := h.hub.NewPartitionClient(partitionID, &azeventhubs.PartitionClientOptions{
 			StartPosition: startPos,
 		})

receiver/azureeventhubreceiver/eventhubhandler_azeventhub_test.go

@@ -225,3 +225,33 @@ func TestPartitionListener_SetErr(t *testing.T) {
 	p.setErr(errors.New("test"))
 	assert.Equal(t, "test", p.err.Error())
 }
+
+func TestGetConsumerGroup(t *testing.T) {
+	testCases := []struct {
+		name          string
+		consumerGroup string
+		expectedGroup string
+	}{
+		{
+			name:          "empty consumer group defaults to $Default",
+			consumerGroup: "",
+			expectedGroup: "$Default",
+		},
+		{
+			name:          "custom consumer group is preserved",
+			consumerGroup: "custom-group",
+			expectedGroup: "custom-group",
+		},
+	}
+
+	for _, test := range testCases {
+		t.Run(test.name, func(t *testing.T) {
+			config := &Config{
+				ConsumerGroup: test.consumerGroup,
+			}
+
+			result := getConsumerGroup(config)
+			assert.Equal(t, test.expectedGroup, result)
+		})
+	}
+}