notify leader election subscribers on leadership state change

[adel121]

What does this PR do?

The leader election engine used to elect the cluster agent leader already provides subscription functionality in order to notify subscribers when the current process becomes the leader.

This PR extends the existing behaviour so that:

• subscribers are notified when the leadership state of the current process changes (leader to follower, follower to leader, etc.)
• includes the new leadership state (LEADER, FOLLOWER, UNKNOWN) in the notification event.

Motivation

Improve code reusability:

• cluster checks dispatching logic needs to know when the leadership state has changed so that it can switch between different behaviours depending on whether the current instance acquired or lost leadership. It also needs to perform some cleanup when leadership is lost. A part of this logic is implemented here.
• language detection patcher needs to depend on whether the current instance is a leader or not to avoid unintended patches. It also needs to perform some actions when leadership status changes. This is not yet implemented, and it will leverage the functionality added in this PR.
• same applies to language detection server (not implemented yet)

The refactor done in this PR aims at unifying how all these use cases deal with the current process leadership state change without needing to manually implement (almost) the same logic every time.

Describe how you validated your changes

Unit tests and e2e tests are already in place:

• we have E2E for admission controller mutating webhooks
• we have unit tests for different relevant components

Teams owning concerned components (like remote config, admission controller webhook) might want to perform some manual QA.

Else, this can be marked as done based on unit tests and E2E.

Possible Drawbacks / Trade-offs

Additional Notes

[cit-pr-commenter]

Regression Detector

Regression Detector Results

Metrics dashboard
Target profiles
Run ID: 045fd55f-88be-4aa3-a16a-1c3dd32d0d1b

Baseline: 06bdd8b
Comparison: 280a71a
Diff

Optimization Goals: ✅ No significant changes detected

Fine details of change detection per experiment

perf	experiment	goal	Δ mean %	Δ mean % CI	trials	links
➖	tcp_syslog_to_blackhole	ingress throughput	+1.33	[+1.26, +1.41]	1	Logs
➖	uds_dogstatsd_to_api_cpu	% cpu utilization	+0.82	[+0.10, +1.54]	1	Logs
➖	quality_gate_idle_all_features	memory utilization	+0.66	[+0.52, +0.80]	1	Logs bounds checks dashboard
➖	file_to_blackhole_1000ms_latency	egress throughput	+0.55	[-0.22, +1.33]	1	Logs
➖	file_to_blackhole_500ms_latency	egress throughput	+0.13	[-0.65, +0.91]	1	Logs
➖	file_to_blackhole_0ms_latency_http1	egress throughput	+0.12	[-0.74, +0.98]	1	Logs
➖	file_to_blackhole_0ms_latency	egress throughput	+0.07	[-0.78, +0.92]	1	Logs
➖	file_to_blackhole_100ms_latency	egress throughput	+0.03	[-0.73, +0.78]	1	Logs
➖	quality_gate_idle	memory utilization	+0.02	[-0.02, +0.07]	1	Logs bounds checks dashboard
➖	file_to_blackhole_0ms_latency_http2	egress throughput	+0.02	[-0.84, +0.88]	1	Logs
➖	uds_dogstatsd_to_api	ingress throughput	+0.01	[-0.08, +0.09]	1	Logs
➖	tcp_dd_logs_filter_exclude	ingress throughput	+0.00	[-0.01, +0.02]	1	Logs
➖	file_to_blackhole_300ms_latency	egress throughput	-0.06	[-0.69, +0.57]	1	Logs
➖	file_to_blackhole_1000ms_latency_linear_load	egress throughput	-0.16	[-0.62, +0.30]	1	Logs
➖	quality_gate_logs	% cpu utilization	-0.32	[-3.24, +2.59]	1	Logs
➖	otel_to_otel_logs	ingress throughput	-0.83	[-1.53, -0.14]	1	Logs
➖	file_tree	memory utilization	-1.96	[-2.09, -1.82]	1	Logs

Bounds Checks: ❌ Failed

perf	experiment	bounds_check_name	replicates_passed	links
❌	file_to_blackhole_0ms_latency_http1	lost_bytes	9/10
✅	file_to_blackhole_0ms_latency	lost_bytes	10/10
✅	file_to_blackhole_0ms_latency	memory_usage	10/10
✅	file_to_blackhole_0ms_latency_http1	memory_usage	10/10
✅	file_to_blackhole_0ms_latency_http2	lost_bytes	10/10
✅	file_to_blackhole_0ms_latency_http2	memory_usage	10/10
✅	file_to_blackhole_1000ms_latency	memory_usage	10/10
✅	file_to_blackhole_1000ms_latency_linear_load	memory_usage	10/10
✅	file_to_blackhole_100ms_latency	lost_bytes	10/10
✅	file_to_blackhole_100ms_latency	memory_usage	10/10
✅	file_to_blackhole_300ms_latency	lost_bytes	10/10
✅	file_to_blackhole_300ms_latency	memory_usage	10/10
✅	file_to_blackhole_500ms_latency	lost_bytes	10/10
✅	file_to_blackhole_500ms_latency	memory_usage	10/10
✅	quality_gate_idle	memory_usage	10/10	bounds checks dashboard
✅	quality_gate_idle_all_features	memory_usage	10/10	bounds checks dashboard
✅	quality_gate_logs	lost_bytes	10/10
✅	quality_gate_logs	memory_usage	10/10

Explanation

Confidence level: 90.00%
Effect size tolerance: |Δ mean %| ≥ 5.00%

Performance changes are noted in the perf column of each table:

• ✅ = significantly better comparison variant performance
• ❌ = significantly worse comparison variant performance
• ➖ = no significant change in performance

A regression test is an A/B test of target performance in a repeatable rig, where "performance" is measured as "comparison variant minus baseline variant" for an optimization goal (e.g., ingress throughput). Due to intrinsic variability in measuring that goal, we can only estimate its mean value for each experiment; we report uncertainty in that value as a 90.00% confidence interval denoted "Δ mean % CI".

For each experiment, we decide whether a change in performance is a "regression" -- a change worth investigating further -- if all of the following criteria are true:

1. Its estimated |Δ mean %| ≥ 5.00%, indicating the change is big enough to merit a closer look.

2. Its 90.00% confidence interval "Δ mean % CI" does not contain zero, indicating that if our statistical model is accurate, there is at least a 90.00% chance there is a difference in performance between baseline and comparison variants.

3. Its configuration does not mark it "erratic".

CI Pass/Fail Decision

✅ Passed. All Quality Gates passed.

• quality_gate_idle_all_features, bounds check memory_usage: 10/10 replicas passed. Gate passed.
• quality_gate_logs, bounds check memory_usage: 10/10 replicas passed. Gate passed.
• quality_gate_logs, bounds check lost_bytes: 10/10 replicas passed. Gate passed.
• quality_gate_idle, bounds check memory_usage: 10/10 replicas passed. Gate passed.