x/telemetry/config: add counters for MCP server usage

[madelinekalil]

Summary

In gopls@v0.20.0, we're launching an experimental MCP server to integrate gopls with MCP-powered coding assistance. We need to add counters for the usage of this new feature, so that we can see adoption.

gopls/mcp-headless:stdio - user starts the mcp server in headless mode running over stdio
gopls/mcp-headless:sse - user starts the mcp server in headless mode running over sse with http
gopls/mcp - user starts the mcp server in attached mode

TODO: is it possible to implement counters for specific tool calls?

Proposed Config Change

xx