Add Telemetry & Metrics Tool with Message Filters (dotnet_server_metrics)

[jongalloway]

Expose lightweight, privacy-respecting metrics and leverage MCP SDK 1.0's Message Filters for telemetry collection.

Tool: dotnet_server_metrics

Returns counts per tool, avg duration ms, cache hit rate.

Implementation

Metrics Accumulator (original scope)

• In-memory metrics collection
• JSON response with per-tool stats
• Reset metrics subcommand
• No PII stored

Message Filters Integration (new — requires MCP SDK 1.0)

MCP SDK 1.0 introduces Message Filters — middleware for intercepting and processing MCP requests/responses. This provides the ideal infrastructure for collecting tool execution telemetry without modifying individual tool implementations.

Planned filters:

• Telemetry filter: Automatically capture tool invocation counts, durations, success/failure rates — feeds the dotnet_server_metrics tool
• Rate limiting filter: Protect against excessive tool invocations
• Validation filter: Pre-validate common parameters (paths, framework TFMs) before tool execution
• Logging filter: Structured logging of all MCP traffic for diagnostics

Implementation:

• Create TelemetryFilter implementing IMessageFilter to capture metrics
• Wire filter into MCP server pipeline in Program.cs
• Feed filter-collected data to the metrics accumulator
• Consider additional filters (rate limiting, validation) as follow-ups

References:

• MCP C# SDK Message Filters
• Related: Adopt telemetry and observability features from SDK v0.6 #324 (adopted SDK v0.6 observability)

Acceptance Criteria

• Metrics accumulator (in-memory)
• JSON response
• Reset metrics subcommand
• No PII stored
• At least one Message Filter wired in for telemetry collection

Dependencies

• Blocked on MCP SDK 1.0 upgrade (see parent tracking issue)

[jongalloway]

This issue is now part of the MCP SDK 1.0 upgrade plan. Message Filters integration has been added to the scope. Tracked by #350.