plugin_proxy: allow metrics type for output plugin proxy

[gracewehner]

This change allows the fluent-bit metrics type to be routed to output proxy plugins such as the golang proxy plugin.

Currently the proxy output plugin registration does not include the out->event_type setting for what types it accepts:

fluent-bit/src/flb_plugin_proxy.c

Lines 364 to 370 in faf7da1

	/* Plugin registration */
	out->type = FLB_OUTPUT_PLUGIN_PROXY;
	out->proxy = proxy;
	out->flags = def->flags;
	out->name = flb_strdup(def->name);
	out->description = def->description;
	mk_list_add(&out->_head, &config->out_plugins);

For output plugins that have no value for event_type, those plugins default to supporting only the logs datatype:

fluent-bit/src/flb_output.c

Lines 605 to 607 in faf7da1

	if (plugin->event_type == 0) {
	instance->event_type = FLB_OUTPUT_LOGS;
	}

The fluent-bit router will enforce datatypes between input and output plugins and drop any data that has the type mismatch:

fluent-bit/src/flb_task.c

Lines 435 to 438 in faf7da1

	/* skip output plugins that don't handle proper event types */
	if (!flb_router_match_type(ic->event_type, o_ins)) {
	continue;
	}

Addresses fluent/fluent-bit-go#75, fluent/fluent-bit-go#77

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Documentation

• Documentation required for this feature

fluent/fluent-bit-docs#1531

Backporting

• Backport to latest stable release.

---

Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.

[gracewehner]

I have a PR for the fluent-bit-go repo fluent/fluent-bit-go#79 that decodes and uses the metric type. It depends on the PR in this repo for metrics to be allowed to flow to the output plugin.

[gracewehner]

Hi @edsiper @leonardo-albertovich @fujimotos @koleini would I be able to get a review on this?

[github-actions]

This PR is stale because it has been open 45 days with no activity. Remove stale label or comment or this will be closed in 10 days.

[gracewehner]

Hi @edsiper @leonardo-albertovich @fujimotos @koleini would you be able to review this still?

[eschabell]

@gracewehner not that there are errors on the tests that need to be addressed.

[edsiper]

@codex review

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Avoid routing metrics to log-only proxy outputs

By unconditionally setting out->event_type to FLB_OUTPUT_LOGS | FLB_OUTPUT_METRICS for all proxy outputs, any proxy output configured for logs will now also accept metrics when a metrics input matches the same output. Because the proxy definition has no way to declare supported event types, log-only proxy plugins (e.g., Go outputs that only implement log decoding) will start receiving metrics chunks they cannot parse, leading to dropped data or flush errors when metrics inputs are present alongside log inputs. This is a behavioral regression for existing deployments that used proxy outputs for logs only and now have metrics enabled; consider making the event type configurable or derived from the proxy/plugin registration instead of forcing metrics support for every proxy output.

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