Added OTEL documentation

[diego-ferrand]

This pull request adds new documentation to the README.md describing how Perfecto MCP uses OpenTelemetry for tracing and metrics. The documentation explains default telemetry behavior, how to disable telemetry, how to configure a custom telemetry collector, and what data is reported.

OpenTelemetry integration documentation:

• Added a new section to README.md detailing how Perfecto MCP reports traces and metrics using OpenTelemetry, including default export settings and service identification.
• Provided instructions for disabling telemetry via the OTEL_SDK_DISABLED environment variable, with examples for both standard and Docker environments.
• Documented how to configure MCP to send telemetry data to a custom OpenTelemetry collector using environment variables, with gRPC and HTTP examples.
• Clarified what information is reported (traces and metrics), and emphasized that security tokens and credentials are not included in telemetry data.
• Explained how MCP links its spans to parent traces if traceparent or tracestate headers are present.