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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Testing
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• [] Example configuration file for the change
• [] Debug log output from testing the change

• [N/A] Attached Valgrind output that shows no leaks or memory corruption was found

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• [N/A] Set ok-package-test label to test for all targets (requires maintainer to do).

Documentation

• Documentation required for this feature

fluent/fluent-bit-docs#1531

Backporting

• Backport to latest stable release.

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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.