feat: add getters for prompt token count and total tokens count in LLMs

[chmjkb]

Description

Currently, there is no way to access the prompt tokens count, which can be useful sometimes. This PR adds two methods to the LLM API, exposing a way to access the mentioned stats.

Introduces a breaking change?

• Yes
• No

Type of change

• Bug fix (change which fixes an issue)
• New feature (change which adds functionality)
• Documentation update (improves or adds clarity to existing documentation)
• Other (chores, tests, code style improvements etc.)

Tested on

• iOS
• Android

Testing instructions

Screenshots

Related issues

Checklist

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have updated the documentation accordingly
• My changes generate no new warnings

Additional notes

[msluszniak]

Personally, I would merge PR with documentation first, because half of this PR will need to be changed after the rebase.