[elastic_agent] "[Elastic Agent] Agent metrics" dashboard: Better filtering for most widgets

There are a number of widgets on this dashboard that can use improved filters to show only the components that are meaningful, below are some categories.

Because https://github.com/elastic/elastic-agent/issues/12577, OTel otput metrics do not appear when the "Agent Hostname" filter is used. So we need to either wait for it to be fixed or, test the dashboard without using its main filter.

## The hostname filter
Even though the filter is labelled as "Agent Hostname" it filters by `agent.name`. Other dashboards like "[Elastic Agent] Input Metrics" label the filter as "Agent Name". The label should match the field used and the other dashboards

## Process metrics

Some of the components listed are running as part of the Otel collector and don't have any data. The CPU Usage widget is already much better than the Memory usage, regardless both list more components than actual OS process.

In this example there are only 3 process being run:
- Elastic Agent
- Elastic Otel Collector
- Log Default (Filebeat)

Probably the easiest fix is to just filter add a filter to ensure the data actually exits, e.g: `filed_name: *`. This ensure only components that actually collect that data will be shown

## Output metrics

Some components that do not send any data are listed in most of the output-related widgets, in this case "elastic-agent" does not send data to an output, "agent-otel-collector" can have multiple outputs, and many other components are running in OTel mode, thus sending data to the internal otelexporter output.

Those widgets need to better show the components actually connected to an output and correct output metrics.

On this screen shot, only "log-default" is running as a process, all other components/inputs are running in Otel mode.