Add MCP metrics

[cmsparks]

This adds 3 event types, which tracks:

• Tool calls by name (with error/no error)
• Session start with userId, clientInfo (i.e. claude/cursor), and clientCapabilities (roots/sampling support)
• Auth user (with error/no error)

To track these events, all you need to use is use the CloudflareMCPServer superclass, which basically adds in all the metrics necessary to track this data as long as you have a metrics binding. All of these metrics are intended to be included in the same server namespace.

We don't track sessionId here, because it's too hard to extract consistently from the McpServer class.

[irvinebroque]

🙌