[FEAT] Add ClientErrorsAsSpanErrors config to otelecho middleware

[Oloruntobi1]

Closes: #6975

This PR introduces a ClientErrorsAsSpanErrors configuration option to the otelecho middleware. When enabled, HTTP 4xx and 5xx status codes are treated as span errors (setting otel.status_code=ERROR and error=true in the root span). The default remains false, where only 5xx codes are errors, preserving existing behavior.

Motivation:
While debugging with Jaeger, I found that traces with 4xx status codes (e.g., 401 Unauthorized) weren’t tagged as errors in the root span, making them hard to filter. For example:

• curl 'http://localhost:16686/api/traces?service=jaeger-nl-demo&tags={%22http.method%22:%22POST%22}' returns a trace with http.status_code=401 at /v1/auth/login, but no error=true in the root span. Child spans like validateCredentials have error=true, but the root span doesn’t reflect this.
• Adding an error filter, like curl 'http://localhost:16686/api/traces?service=jaeger-nl-demo&tags={%22http.method%22:%22POST%22,%22error%22:%22true%22}' or ...?tags={"http.method":"POST","error":"route"}, returns no results because the middleware doesn’t mark 4xx as errors.
• Even combining filters like http.route=/v1/auth/login with "error":"true" fails for the same reason.

This disconnect complicates debugging, as users expect error filters to catch application-level failures (e.g., authentication errors). The new option lets users opt into treating 4xx as errors, aligning the root span with downstream error signals.

Changes:

• Added ClientErrorsAsSpanErrors to config.go with a WithClientErrorsAsSpanErrors setter.
• Modified echo.go to set codes.Error for 4xx/5xx when enabled.
• Default behavior unchanged (only 5xx are errors).

Impact:

• With ClientErrorsAsSpanErrors=true, a 401 response at /v1/auth/login now includes otel.status_code=ERROR and error=true in the root span, making it visible in Jaeger error queries.
• Tested locally with Jaeger; queries like tags={"error":"true"} now return 4xx traces as expected.

Testing:

• No unit tests added yet—existing status code tests may cover this, but I can add a specific 4xx case if requested.

Feedback welcome!

[linux-foundation-easycla]

• ✅login: Oloruntobi1 / (853bc09)

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