Add option to specify metadata columns in CSV loader (#11576)

#### Description
This PR adds the option to specify additional metadata columns in the
CSVLoader beyond just `Source`.

The current CSV loader includes all columns in `page_content` and if we
want to have columns specified for `page_content` and `metadata` we have
to do something like the below.:
```
csv = pd.read_csv(
        "path_to_csv"
    ).to_dict("records")

documents = [
        Document(
            page_content=doc["content"],
            metadata={
                "last_modified_by": doc["last_modified_by"],
                "point_of_contact": doc["point_of_contact"],
            }
        ) for doc in csv
    ]
```
#### Usage
Example Usage:
```
csv_test  =  CSVLoader(
      file_path="path_to_csv", 
      metadata_columns=["last_modified_by", "point_of_contact"]
 )
```
Example CSV:
```
content, last_modified_by, point_of_contact
"hello world", "Person A", "Person B"
```

Example Result:
```
Document {
 page_content: "hello world"
 metadata: {
 row: '0',
 source: 'path_to_csv',
 last_modified_by: 'Person A',
 point_of_contact: 'Person B',
 }
```

---------

Co-authored-by: Ben Chello <bchello@dropbox.com>
Co-authored-by: Bagatur <baskaryan@gmail.com>

libs/langchain/langchain/document_loaders/csv_loader.py

@@ -1,6 +1,6 @@
 import csv
 from io import TextIOWrapper
-from typing import Any, Dict, List, Optional
+from typing import Any, Dict, List, Optional, Sequence
 
 from langchain.docstore.document import Document
 from langchain.document_loaders.base import BaseLoader
@@ -36,6 +36,7 @@ class CSVLoader(BaseLoader):
         self,
         file_path: str,
         source_column: Optional[str] = None,
+        metadata_columns: Sequence[str] = (),
         csv_args: Optional[Dict] = None,
         encoding: Optional[str] = None,
         autodetect_encoding: bool = False,
@@ -46,6 +47,7 @@ class CSVLoader(BaseLoader):
             file_path: The path to the CSV file.
             source_column: The name of the column in the CSV file to use as the source.
               Optional. Defaults to None.
+            metadata_columns: A sequence of column names to use as metadata. Optional.
             csv_args: A dictionary of arguments to pass to the csv.DictReader.
               Optional. Defaults to None.
             encoding: The encoding of the CSV file. Optional. Defaults to None.
@@ -53,6 +55,7 @@ class CSVLoader(BaseLoader):
         """
         self.file_path = file_path
         self.source_column = source_column
+        self.metadata_columns = metadata_columns
         self.encoding = encoding
         self.csv_args = csv_args or {}
         self.autodetect_encoding = autodetect_encoding
@@ -85,9 +88,9 @@ class CSVLoader(BaseLoader):
 
     def __read_file(self, csvfile: TextIOWrapper) -> List[Document]:
         docs = []
+
         csv_reader = csv.DictReader(csvfile, **self.csv_args)  # type: ignore
         for i, row in enumerate(csv_reader):
-            content = "\n".join(f"{k.strip()}: {v.strip()}" for k, v in row.items())
             try:
                 source = (
                     row[self.source_column]
@@ -98,7 +101,17 @@ class CSVLoader(BaseLoader):
                 raise ValueError(
                     f"Source column '{self.source_column}' not found in CSV file."
                 )
+            content = "\n".join(
+                f"{k.strip()}: {v.strip()}"
+                for k, v in row.items()
+                if k not in self.metadata_columns
+            )
             metadata = {"source": source, "row": i}
+            for col in self.metadata_columns:
+                try:
+                    metadata[col] = row[col]
+                except KeyError:
+                    raise ValueError(f"Metadata column '{col}' not found in CSV file.")
             doc = Document(page_content=content, metadata=metadata)
             docs.append(doc)