Instrument Otel metrics

[Shweta-Deshpande]

This PR adds OpenTelemetry metrics instrumentation to the MCP server so we can observe tool-call volume, latency, and failures in Streamable HTTP mode.

• Added metrics config and recording logic in pkg/client/metrics.go
• Added OTel metrics setup/initialization in cmd/terraform-mcp-server/main.go
• Wired metrics into server tool-call hooks in cmd/terraform-mcp-server/main.go
• Passed metrics config through server startup paths (HTTP mode uses configured metrics; stdio remains with empty config)
• Added unit tests for metrics behavior:
  pkg/client/metrics_test.go
  cmd/terraform-mcp-server/metrics_test.go

Metrics Added:
mcp_tool_calls_total (Counter)
mcp_tool_errors_total (Counter)
mcp_tool_duration_seconds (Histogram)

PCI review checklist

• I have documented a clear reason for, and description of, the change I am making.

• If applicable, I've documented a plan to revert these changes if they require more than reverting the pull request.

• If applicable, I've documented the impact of any changes to security controls.

  Examples of changes to security controls include using new access control methods, adding or removing logging pipelines, etc.