Collect telemetry on MCP tool output sizes

**Problem**

To avoid blowing out the LM's context window, when the output size of a tool exceeds 20 KB the agent (Copilot CLI, Claude Code, etc.) will generally _not_ provide the output to the LM as-is. It will instead find some way of cutting down the information, possibly including:
- truncating the output
- deleting data from the middle of the output
- using AI to summarize the data down to a smaller size
- spilling the data to a file, and providing the LM the path to the file

In all cases information is likely to be lost, in the sense that it doesn't make it from the tool back to the LM.

Currently we do not know how often this happens, so we cannot design an appropriate response.

**Solution**

Add telemetry to every tool call to capture the size of the tool output.