multi_output: fix metrics name

[Watson1978]

Which issue(s) this PR fixes:
Fixes #

What this PR does / why we need it:
In multi_output, two metrics are created with emit_records.

fluentd/lib/fluent/plugin/multi_output.rb

Lines 94 to 95 in 0b72a77

	@emit_count_metrics = metrics_create(namespace: "fluentd", subsystem: "multi_output", name: "emit_records", help_text: "Number of count emits")
	@emit_records_metrics = metrics_create(namespace: "fluentd", subsystem: "multi_output", name: "emit_records", help_text: "Number of emit records")

Since it is not possible to distinguish between them, this PR will fix the name.

Docs Changes:
Not needed.

Release Note:
The same as the title.

[daipom]

Thanks!

@cosmo0920
If you know anything, could you please let me know?
Could this name change potentially affect users?
I don't know much about metrics...
As for Fluentd's internal logic, it does not seem to be affected.

[cosmo0920]

It is no relevant issues for now. Thanks for the good catch. 👍

[daipom]

Thanks!

[cosmo0920]

The local metrics plugin should not handle the name of metrics. This is only for affected for metrics_cmetrics plugin which was intended to handle the internal metrics in Fluentd with cmetrics mechanism.
https://github.com/chronosphereio/calyptia-fluent-plugin-metrics-cmetrics

Are y'all interested in handling metrics with the current cmetrics?
This is because we had changed the API especially for msgpack encoding/decoding. So, if the using cmetrics library upgraded, we need to publish another gem like as cmetrics2 in rubygems due to incompatibilities for older version of Fluent Bit.
https://rubygems.org/gems/cmetrics

This should be using the old version of cmetrics which was in older fluent-bit.

For now, I have no idea to fix the incompatibilities and how to handle newer version of cmetrics msgpack payloads and the older version of payloads in the same C extension context.

Currently, cmetrics had been reached v1.0.0 milestone. So, it's suitable time to upgrade the cmetrics ruby binding here:
https://github.com/chronosphereio/calyptia-cmetrics-ruby

[daipom]

Thanks!
For now, I just wanted to know the impact of changing a metric name.
I would be happy to improve Fluentd metrics and observability, but I don't have much time to focus on it for a while...
(By the way, @Watson1978 made https://github.com/fluent-plugins-nursery/fluent-plugin-opentelemetry, so Fluentd supports OTel at last.)