docs(pallas): document sampling parameters and Prometheus metrics

Add two new sections to the Pallas documentation:

- Sampling parameters: explain that temperature/top_p/top_k are
  configured via the fast-agent decorator's `request_params`, with a
  provider support matrix and a note on Claude Opus 4.7 stripping these
  params in favor of `output_config.effort`.
- Metrics: document the Prometheus `/metrics` endpoint exposed on the
  registry port, including scrape config, full metrics reference table,
  and notes on where each metric is captured.

pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "pallas-mcp"
-version = "0.2.1"
+version = "0.2.2"
 description = "FastAgent MCP Bridge — generic runtime for serving FastAgent agents over StreamableHTTP"
 requires-python = ">=3.13.5"
 dependencies = [