write-tee: fix preferred backend selection

[pracucci]

What this PR does

In the test cluster we're using, we're currently configuring the mirrored backend == amplified backend == preferred backend. This means that both the mirrored and amplified backend are both marked as preferred, which means that when amplifying the traffic, the amplification is skipped because we have a check to ensure we never amplify to the preferred backend (because that's the sync one).

In this PR I'm changing the preferred backend logic. After rolling out this change, the traffic doubled in our test cluster (we configure it with amplification factor == 1):

Which issue(s) this PR fixes or relates to

Fixes #

Checklist

• Tests updated.
• Documentation added.
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]. If changelog entry is not needed, please add the changelog-not-needed label to the PR.
• about-versioning.md updated with experimental features.

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

^{Bugbot Autofix is ON. A Cloud Agent has been kicked off to fix the reported issue.}

[cursor]

Bugbot Autofix prepared fixes for 1 of the 1 bugs found in the latest run.

• ✅ Fixed: Unused idx parameter in parseBackendEndpoint after refactor
  • Removed the unused idx parameter from parseBackendEndpoint and cleaned up the misleading baseIdx variable and its comment at both call sites.

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@cursor push b042879f33

Preview (b042879f33)

diff --git a/tools/writetee/proxy.go b/tools/writetee/proxy.go
--- a/tools/writetee/proxy.go
+++ b/tools/writetee/proxy.go
@@ -141,8 +141,8 @@
 	// Parse mirrored backend endpoints (comma separated).
 	if cfg.BackendMirroredEndpoints != "" {
 		parts := strings.Split(cfg.BackendMirroredEndpoints, ",")
-		for idx, part := range parts {
-			backend, err := p.parseBackendEndpoint(part, idx, BackendTypeMirrored)
+		for _, part := range parts {
+			backend, err := p.parseBackendEndpoint(part, BackendTypeMirrored)
 			if err != nil {
 				return nil, err
 			}
@@ -155,9 +155,8 @@
 	// Parse amplified backend endpoints (comma separated).
 	if cfg.BackendAmplifiedEndpoints != "" {
 		parts := strings.Split(cfg.BackendAmplifiedEndpoints, ",")
-		baseIdx := len(p.backends) // offset for numeric preferred backend matching
-		for idx, part := range parts {
-			backend, err := p.parseBackendEndpoint(part, baseIdx+idx, BackendTypeAmplified)
+		for _, part := range parts {
+			backend, err := p.parseBackendEndpoint(part, BackendTypeAmplified)
 			if err != nil {
 				return nil, err
 			}
@@ -213,7 +212,7 @@
 	return p, nil
 }
 
-func (p *Proxy) parseBackendEndpoint(endpoint string, idx int, backendType BackendType) (ProxyBackend, error) {
+func (p *Proxy) parseBackendEndpoint(endpoint string, backendType BackendType) (ProxyBackend, error) {
 	// Skip empty ones.
 	endpoint = strings.TrimSpace(endpoint)
 	if endpoint == "" {