[exporter/loadbalancing] Update k8sresolver handler to properly manag…

…e update events (open-telemetry#36505)

<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->
#### Description
The load balancing exporter's k8sresolver was not handling update events
properly. The `callback` function was being executed after cleanup of
old endpoints and also after adding new endpoints. This causes exporter
churn in the case of an event in which the lists contain shared
elements. See the
[documentation](https://pkg.go.dev/k8s.io/client-go/tools/cache#ResourceEventHandler)
for examples where the state might change but the IP Addresses would
not, including the regular re-list events that might have zero changes.

<!-- Issue number (e.g. open-telemetry#1234) or full URL to issue, if applicable. -->
#### Link to tracking issue
Fixes
open-telemetry#35658
May be related to
open-telemetry#35810
as well.

<!--Describe what testing was performed and which tests were added.-->
#### Testing
Added tests for no-change onChange call.

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

.chloggen/single-callback-k8sresolver.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: loadbalancingexporter
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: The k8sresolver in loadbalancingexporter was triggering exporter churn in the way the change event was handled.
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [35658]
+
+# (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: []