(fleet) small improvements to the mirroring logic

[arbll]

This PR adds a few improvements to the mirroring logic:

• Don't try to parse manifest JSON on non-200 to avoid hard to understand logs with JSON parsing
• Set the request host header properly, fixing tracing and potentially mirrors behind a LB

Describe how to test/QA your changes

TODO

[cit-pr-commenter]

Regression Detector

Regression Detector Results

Metrics dashboard
Target profiles
Run ID: b83f2a77-225a-4f31-aedf-ca6f43f3315b

Baseline: a768373
Comparison: 3b16e19
Diff

Optimization Goals: ❌ Significant changes detected

perf	experiment	goal	Δ mean %	Δ mean % CI	trials	links
✅	pycheck_lots_of_tags	% cpu utilization	-5.15	[-8.51, -1.78]	1	Logs

Fine details of change detection per experiment

perf	experiment	goal	Δ mean %	Δ mean % CI	trials	links
➖	quality_gate_idle_all_features	memory utilization	+2.49	[+2.38, +2.60]	1	Logs bounds checks dashboard
➖	uds_dogstatsd_to_api_cpu	% cpu utilization	+1.27	[+0.55, +2.00]	1	Logs
➖	file_to_blackhole_500ms_latency	egress throughput	+0.51	[-0.25, +1.27]	1	Logs
➖	file_to_blackhole_1000ms_latency_linear_load	egress throughput	+0.46	[+0.01, +0.92]	1	Logs
➖	tcp_syslog_to_blackhole	ingress throughput	+0.21	[+0.15, +0.27]	1	Logs
➖	file_to_blackhole_0ms_latency	egress throughput	+0.05	[-0.80, +0.90]	1	Logs
➖	uds_dogstatsd_to_api	ingress throughput	+0.03	[-0.07, +0.13]	1	Logs
➖	file_to_blackhole_100ms_latency	egress throughput	+0.01	[-0.67, +0.70]	1	Logs
➖	tcp_dd_logs_filter_exclude	ingress throughput	+0.00	[-0.01, +0.01]	1	Logs
➖	file_to_blackhole_1000ms_latency	egress throughput	-0.09	[-0.88, +0.70]	1	Logs
➖	otel_to_otel_logs	ingress throughput	-0.09	[-0.77, +0.58]	1	Logs
➖	file_tree	memory utilization	-0.15	[-0.28, -0.01]	1	Logs
➖	file_to_blackhole_300ms_latency	egress throughput	-0.16	[-0.80, +0.47]	1	Logs
➖	quality_gate_idle	memory utilization	-0.21	[-0.25, -0.17]	1	Logs bounds checks dashboard
➖	basic_py_check	% cpu utilization	-4.68	[-8.36, -1.00]	1	Logs
✅	pycheck_lots_of_tags	% cpu utilization	-5.15	[-8.51, -1.78]	1	Logs

Bounds Checks: ❌ Failed

perf	experiment	bounds_check_name	replicates_passed	links
❌	file_to_blackhole_500ms_latency	lost_bytes	8/10
✅	file_to_blackhole_0ms_latency	lost_bytes	10/10
✅	file_to_blackhole_0ms_latency	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	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

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

[agent-platform-auto-pr]

Test changes on VM

Use this command from test-infra-definitions to manually test this PR changes on a VM:

inv create-vm --pipeline-id=50055457 --os-family=ubuntu

Note: This applies to commit 3b16e19