fix(metricprovider): respect --logformat json for metric plugin process logger

[sujanchalla0510]

Problem

Fixes #4408.

The metric provider plugin client (metricproviders/plugin/client/client.go) builds its go-plugin ClientConfig without ever setting Logger:

m.pluginClient[pluginName] = goPlugin.NewClient(&goPlugin.ClientConfig{
    HandshakeConfig: handshakeConfig,
    Plugins:         pluginMap,
    Cmd:             exec.Command(pluginPath, args...),
    Managed:         true,
})

hashicorp/go-plugin's own Start() explicitly falls back to its own hardcoded, unstructured hclog logger whenever Logger is nil:

// hashicorp/go-plugin client.go
if config.Logger == nil {
    config.Logger = hclog.New(&hclog.LoggerOptions{
        Output: hclog.DefaultOutput,
        Level:  hclog.Trace,
        Name:   "plugin",
    })
}

This means every log line related to the metric plugin's handshake/lifecycle, and the relayed stderr of the plugin subprocess itself, is always emitted in go-plugin's own fixed text format (argo-rollouts 2025-08-18T09:53:56.003Z [DEBUG] plugin.my-plugin: ...) — regardless of the controller's own --logformat json flag. Every other controller log line respects --logformat; this is the one place that doesn't, which breaks structured-log parsing/ingestion pipelines built around the controller's JSON output.

Verification the bug is real and current

Confirmed directly against current master (4e6a27986, 2026-08-25):

• metricproviders/plugin/client/client.go's ClientConfig literal has no Logger field, exactly as described in the issue.
• go-plugin (v1.8.0, this repo's current pinned version) still has the if config.Logger == nil { ... } fallback quoted above (hashicorp/go-plugin@v1.8.0/client.go:417).
• No prior PR or commit touches this file to address logger configuration (git log -- metricproviders/plugin/client/client.go shows no related change).
• The same pattern (no Logger set) also exists in rollout/trafficrouting/plugin/client/client.go and rollout/steps/plugin/client/client.go, which are out of scope for this PR — it's scoped narrowly to the metric provider plugin client, matching the issue title/report. Happy to open follow-up PRs for the other two plugin types if maintainers want the same treatment there.

Fix

Added newPluginLogger() (and a testable newPluginLoggerWithOutput() helper) in metricproviders/plugin/client/client.go:

• If the controller's standard logrus logger is configured with logrus.JSONFormatter (i.e. the controller was started with --logformat json, which cmd/rollouts-controller/main.go applies via log.SetFormatter(...) on the same package-level logrus standard logger), build an hclog.Logger with JSONFormat: true, keeping every other option (Trace level, "plugin" name, hclog.DefaultOutput) identical to go-plugin's own default so nothing else about the logger's behavior changes.
• Otherwise, return nil, which is exactly what was passed implicitly before this change — go-plugin falls back to its own default (unstructured) logger, so behavior for the default/text-format case (the overwhelming majority of current deployments) is completely unchanged.

This is a minimal, additive, non-breaking change: it only changes plugin logger output format, never plugin behavior, and only takes effect when the operator has already opted into --logformat json.

Test

Added metricproviders/plugin/client/client_test.go (this package had no test file at all before this PR):

• newPluginLogger() returns nil when the controller's logger is the default logrus.TextFormatter — preserving go-plugin's own default logger.
• newPluginLoggerWithOutput() (used to capture output without writing to the real hclog.DefaultOutput/stderr in the test) returns a non-nil logger that emits a single valid JSON object per log line when the controller's logger is logrus.JSONFormatter, verified by unmarshaling the captured output and asserting the @message field.

Revert-and-reconfirm proof: reverted metricproviders/plugin/client/client.go alone (kept the new test file and go.mod) and re-ran the test:

metricproviders/plugin/client/client_test.go:23:13: undefined: newPluginLogger
metricproviders/plugin/client/client_test.go:31:13: undefined: newPluginLoggerWithOutput
FAIL	github.com/argoproj/argo-rollouts/metricproviders/plugin/client [build failed]

Restored the fix and confirmed:

--- PASS: TestNewPluginLoggerRespectsControllerLogFormat (0.00s)
    --- PASS: .../default_text_logformat_leaves_go-plugin's_own_default_logger_untouched (0.00s)
    --- PASS: .../json_logformat_produces_a_JSON_plugin_logger (0.00s)
PASS
ok  	github.com/argoproj/argo-rollouts/metricproviders/plugin/client	0.65s

Full verification before submission:

• go build ./... — clean.
• go vet ./... — clean.
• go test ./metricproviders/... — all packages pass except a pre-existing, unrelated failure in metricproviders/datadog (TestRunSuite/TestRunSuiteV2), which I confirmed also fails identically on unmodified upstream/master before this change (JSON error-message field-casing mismatch, apparently due to a newer Go toolchain's stdlib encoding/json error text in my local environment — unrelated to this PR's package).
• gofmt -l metricproviders/plugin/client/ — no output (clean).

Scope note

go.mod moves github.com/hashicorp/go-hclog from an indirect to a direct requirement (it was already present transitively via go-plugin, version unchanged at v1.6.3) since this PR now imports it directly. No other dependency changes.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[github-actions]

Published E2E Test Results

  4 files    4 suites   4h 10m 58s ⏱️
149 tests 137 ✅  7 💤 5 ❌
602 runs  568 ✅ 28 💤 6 ❌

For more details on these failures, see this check.

Results for commit ec0584f.

[codecov]

Codecov Report

❌ Patch coverage is 91.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 85.17%. Comparing base (4e6a279) to head (ec0584f).

Files with missing lines	Patch %	Lines
metricproviders/plugin/client/client.go	91.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5000      +/-   ##
==========================================
+ Coverage   85.15%   85.17%   +0.01%     
==========================================
  Files         166      166              
  Lines       19453    19465      +12     
==========================================
+ Hits        16566    16580      +14     
+ Misses       2032     2031       -1     
+ Partials      855      854       -1     

Flag	Coverage Δ
e2e	53.09% <0.00%> (+0.06%)	⬆️
unit-tests	81.65% <91.66%> (+0.02%)	⬆️

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[github-actions]

Published Unit Test Results

2 628 tests   2 628 ✅  3m 29s ⏱️
  131 suites      0 💤
    1 files        0 ❌

Results for commit ec0584f.