Golang x/tools/gopls: add counters for MCP server usage

In gopls@v0.20.0, we're launching an experimental MCP server to integrate gopls with MCP-powered coding assistance (beyond the default integration already done via LSP by things like Copilot): #73580. We should be sure to add counters for the usage of this new feature, so that we can see adoption.

This issue just tracks the instrumentation of this telemetry, which must be done before the v0.20.0 release. A separate proposal can propose collection.

CC @h9jiang @madelinekalil

Comment From: gabyhelp

Related Issues

• x/tools/gopls: expose an off-by-default MCP server from gopls #73580

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