[Repo Assist] fix(cmd): apply OTEL_SERVICE_NAME env var override to tracing config

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Problem

OTEL_SERVICE_NAME environment variable was silently ignored when configuring the OTLP tracing service name.

Root cause

In internal/cmd/root.go, the three tracing flags were applied inconsistently:

// ✅ Endpoint: correctly applies env-var override
if otlpEndpoint != "" || cmd.Flags().Changed("otlp-endpoint") {
    ensureTracingConfig(cfg).Endpoint = otlpEndpoint
}
// ❌ Service name: only applied when flag was explicitly set on CLI
if cmd.Flags().Changed("otlp-service-name") {
    ensureTracingConfig(cfg).ServiceName = otlpServiceName
}

When OTEL_SERVICE_NAME=my-service is set in the environment, otlpServiceName picks up the value via the flag's default (envutil.GetEnvString("OTEL_SERVICE_NAME", ...)), but cmd.Flags().Changed("otlp-service-name") is false since no explicit CLI flag was passed. The env var value was therefore never written to the config, and resolveServiceName fell back to "mcp-gateway".

The config documentation explicitly states OTEL_SERVICE_NAME — overrides ServiceName, so this is a bug.

Fix

Use applyFlagOrEnv for both otlp-endpoint and otlp-service-name, consistent with the payload flag block (lines 256–258). Pre-fetch the TracingConfig pointer once to avoid repeated ensureTracingConfig(cfg) calls:

tc := ensureTracingConfig(cfg)
applyFlagOrEnv(cmd, "otlp-endpoint", &tc.Endpoint, otlpEndpoint, "")
applyFlagOrEnv(cmd, "otlp-service-name", &tc.ServiceName, otlpServiceName, config.DefaultTracingServiceName)
if cmd.Flags().Changed("otlp-sample-rate") {
    tc.SampleRate = &otlpSampleRate
}

otlp-sample-rate retains the Changed-only check since it has no env var override.

Tests added

Two new tests in internal/cmd/flags_tracing_test.go:

• TestApplyTracingFlags_ServiceNameEnvVar — verifies OTEL_SERVICE_NAME propagates to tracing config when flag is not explicitly set
• TestApplyTracingFlags_ServiceNameDefaultDoesNotOverrideConfig — verifies TOML config service name is preserved when env var is unset and flag is unchanged

Test Status

⚠️ Infrastructure limitation: The sandbox network firewall blocks downloading Go 1.25.0 (required by go.mod). Build and tests could not be executed locally. The changes are syntactically verified; the logic follows the same pattern as the existing applyFlagOrEnv calls on lines 256–258 which are already covered by passing tests.

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Pull request overview

This PR fixes a bug in the gateway CLI config merge where OTEL_SERVICE_NAME (picked up via the --otlp-service-name flag default) was not being written into cfg.Gateway.Tracing.ServiceName unless the CLI flag was explicitly passed, causing tracing to fall back to config.DefaultTracingServiceName even when the env var was set.

Changes:

• Update tracing flag application in internal/cmd/root.go to use applyFlagOrEnv for otlp-service-name (and refactor to reuse a single tracing config pointer).
• Add tests to verify OTEL_SERVICE_NAME propagates into config when the flag is not explicitly set, and that defaults do not overwrite an existing config value.

Show a summary per file

File	Description
internal/cmd/root.go	Uses applyFlagOrEnv to correctly apply env-var overrides to tracing config (service name).
internal/cmd/flags_tracing_test.go	Adds regression tests covering env-var override propagation and default non-overwrite behavior.

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• Files reviewed: 2/2 changed files
• Comments generated: 2