[APM] Mobile APM Most used widget

[boriskirov]

Add a most used widget in the Mobile APM Service overview page:
Initial proposal

Deliver the following chars using Lens

• Add new UI component in the mobile APM agent's service overview page highlighting 'Most used device', 'Most used OS', etc., similar to what has been shown in the mock-up located here

• We've agreed to investigate if we can visualize the information in the way we have the proposed design, if not we can pick something like a pie-chart or something else.

• We don't need to add links in the widget for now

• Investigate lens embeddable as part of this ticket

Related to:
https://github.com/elastic/apm-dev/issues/823
Figma Design explorations
#143498

[elasticmachine]

Pinging @elastic/apm-ui (Team:APM)

[gbamparop]

@boriskirov I believe we have discussed to investigate introducing lens embeddable for this and use a pie chart or something similar?

[boriskirov]

@gbamparop It is part of the ticket description now, once you conduct the research and have the finding, let's schedule a session and discuss things.

[kpatticha]

findings using lens embeddable:

Pros

1. Lens embeddable allows us to create new charts fast
   i. no ned to create new endpoint
   ii. no need to create custom UI charts using EuiChart
2. consistency across the app(s)
3. additional actions come for free (in the example didn't work, I might miss a configuration)
   i. inspect
   ii. explore data in discover

Cons

1. Limited visualisation options - we can use only what lens offers
2. Limited data processing
3. in the example, I've noticed the embeddable charts take a bit longer to render compare to the custom ones.

Next steps:

1. create PR with the lens embeddable in use to gather feedback from the engineering team
2. meanwhile @boriskirov we'll play with lens to find another way to present the most used values
3. I'll reach out to agent-mobile team for more realistic data or create a synthrace scenario if possible

[kpatticha]

There is another disadvantage of using lens.

In the service overview page the user has the option to enable/disable the comparison feature.
Currently, neither map nor most users pie charts display comparison data and as far I know, lens do not support that.

This is something we need to take into consideration for the next cycle.

Thanks @dgieselaar for catching it 👍

cc @gbamparop @boriskirov