revert(prom): keep the remote write v1 decode buffer until the write finishes

This reverts commit 8232c5b3bb.

The v1 decoder fabricates `&'static [u8]` pointing into its own decode buffer
(prom_remote_write/types.rs), so the compiler checks nothing about that
buffer's lifetime. Holding the builder until the handler returns is what keeps
the decoder safe by construction; releasing it early made that safety depend on
every consumer copying out of the buffer, which holds today but nothing
enforces.

Document the requirement at the binding instead.

Signed-off-by: Dennis Zhuang <killme2008@gmail.com>

src/servers/src/http/prom_store.rs

@@ -175,17 +175,15 @@ async fn remote_write_v1(
         processor.set_pipeline(pipeline_handler, query_ctx.clone(), pipeline_def);
     }
 
-    let mut decoded =
-        decode_remote_write_request(is_zstd, body, prom_validation_mode, &mut processor)?;
+    // Hold `req` until the write finishes. The v1 decoder fabricates
+    // `&'static [u8]` into its own decode buffer, so nothing about that
+    // buffer's lifetime is checked by the compiler.
+    let mut req = decode_remote_write_request(is_zstd, body, prom_validation_mode, &mut processor)?;
 
-    // Rows and pipeline values own their data; the decode buffer need not span downstream awaits.
     let req = if processor.use_pipeline {
-        drop(decoded);
         processor.exec_pipeline().await?
     } else {
-        let req = decoded.as_insert_requests();
-        drop(decoded);
-        req
+        req.as_insert_requests()
     };
     let batches = into_prom_write_batches(req, query_ctx);