multi_output: fix metrics name (#4979)

**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`.

https://github.com/fluent/fluentd/blob/0b72a77d23a667c2c5c648e457e16fc8835c2134/lib/fluent/plugin/multi_output.rb#L94-L95

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.

Signed-off-by: Shizuo Fujita <fujita@clear-code.com>

Author: Watson1978

lib/fluent/plugin/multi_output.rb

@@ -91,7 +91,7 @@ def configure(conf)
         super
 
         @num_errors_metrics = metrics_create(namespace: "fluentd", subsystem: "multi_output", name: "num_errors", help_text: "Number of count num errors")
-        @emit_count_metrics = metrics_create(namespace: "fluentd", subsystem: "multi_output", name: "emit_records", help_text: "Number of count emits")
+        @emit_count_metrics = metrics_create(namespace: "fluentd", subsystem: "multi_output", name: "emit_count", 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")
         @emit_size_metrics =  metrics_create(namespace: "fluentd", subsystem: "multi_output", name: "emit_size", help_text: "Total size of emit events")
         @enable_size_metrics = !!system_config.enable_size_metrics