add metal to ecosystem (langchain-ai#3613)

Author: softboyjimbo

docs/_static/MetalDash.png

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+# Metal
+
+This page covers how to use [Metal](https://getmetal.io) within LangChain.
+
+## What is Metal?
+
+Metal is a  managed retrieval & memory platform built for production. Easily index your data into `Metal` and run semantic search and retrieval on it.
+
+![Metal](../_static/MetalDash.png)
+
+## Quick start
+
+Get started by [creating a Metal account](https://app.getmetal.io/signup).
+
+Then, you can easily take advantage of the `MetalRetriever` class to start retrieving your data for semantic search, prompting context, etc. This class takes a `Metal` instance and a dictionary of parameters to pass to the Metal API.
+
+```python
+from langchain.retrievers import MetalRetriever
+from metal_sdk.metal import Metal
+
+
+metal = Metal("API_KEY", "CLIENT_ID", "INDEX_ID");
+retriever = MetalRetriever(metal, params={"limit": 2})
+
+docs = retriever.get_relevant_documents("search term")
+```