Add Column Header to the exported CSV from query command (#1313)

# Issue
#1285

The CSV exported from the `mf query <metric> --csv <csv-file-name>`
command does not contain the column header. The CSV thus does not make
too much sense.

# Fix
- The code
[here](https://github.com/dbt-labs/metricflow/blob/main/dbt-metricflow/dbt_metricflow/cli/main.py#L344)
is writing data to the CSV file. The column names aren't getting added
to the file.
- With this PR, we are adding the column names returned from
`df.column_names` to the CSV to fix the issue.

.changes/unreleased/Fixes-20240716-090114.yaml

@@ -0,0 +1,6 @@
+kind: Fixes
+body: Add Column header to the exported CSV from query command
+time: 2024-07-16T09:01:14.102978+05:30
+custom:
+    Author: saurabh0402
+    Issue: 1285,1313

dbt-metricflow/dbt_metricflow/cli/main.py

@@ -340,6 +340,7 @@ def query(
         elif csv is not None:
             # csv is a LazyFile that is file-like that works in this case.
             csv_writer = csv_module.writer(csv)
+            csv_writer.writerow(df.column_names)
             for row in df.rows:
                 csv_writer.writerow(row)
             click.echo(f"🖨 Successfully written query output to {csv.name}")