AI Gateway 2.1: ADD Model, MCP Server, and Agent attribution to Konnect Analytics

## Overview

AI Gateway 2.1 adds Model, MCP Server, and Agent attribution to Konnect Analytics, allowing users to analyze AI traffic by the corresponding AI Gateway entity.

## Scope

Update AI Gateway Analytics docs to cover:

* Model, MCP Server, and Agent IDs/names as Analytics dimensions.
* Filtering and grouping by these entities.
* Example use cases: token usage by Model, tool activity by MCP Server, and traffic/errors by Agent.
* Behavior when entity attribution isn't available.

## DoD

* New entity dimensions are documented in https://developer.konghq.com/observability/explorer/ (most probably) along with other relevant places.
* Filtering/grouping and representative use cases are covered.
* Existing Analytics docs use AI Gateway 2.1 entity terminology.

Start with the AI Gateway Analytics documentation at https://developer.konghq.com/observability/explorer/ and locate the existing dimensions and filtering guidance. Document Model, MCP Server, and Agent IDs/names, grouping, filtering, representative use cases, and behavior when attribution is unavailable. Confirm related Analytics pages use the AI Gateway 2.1 terminology.

Written by the indexing model from the issue text.