Add around_request hook for request instrumentation

[koic]

Motivation and Context

instrumentation_callback fires after request execution, making it impossible to wrap requests with Application Performance Monitoring (APM) spans (e.g. Datadog tracing). This adds around_request, which wraps request handling and allows executing code before and after each request.

For example, to wrap requests with a Datadog trace:

MCP.configure do |config|
  config.around_request = ->(data, &request_handler) {
    Datadog::Tracing.trace("mcp.#{data[:method]}") do
      request_handler.call
    end
  }
end

instrumentation_callback is soft-deprecated in favor of around_request.

How Has This Been Tested?

Added tests for around_request in configuration, instrumentation, and server test files. All existing tests continue to pass.

Breaking Changes

around_request itself is optional and defaults to a pass-through, and instrumentation_callback continues to work as before.

However, instrument_call now accepts a server_context: keyword and its block is invoked without arguments (previously yield block passed the block Proc as an argument). This affects anyone mixing MCP::Instrumentation into their own classes, though the public Server API is unchanged.

Resolves #302.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

[atesgoral]

Nice!