pageserver/page_api: add binary Protobuf descriptor (#11968)

## Problem

A binary Protobuf schema descriptor can be used to expose an API
reflection service, which in turn allows convenient usage of e.g.
`grpcurl` against the gRPC server.

Touches #11728.

## Summary of changes

* Generate a binary schema descriptor as
`pageserver_page_api::proto::FILE_DESCRIPTOR_SET`.
* Opportunistically rename the Protobuf package from `page_service` to
`page_api`.

pageserver/page_api/build.rs

@@ -1,7 +1,13 @@
+use std::env;
+use std::path::PathBuf;
+
+/// Generates Rust code from .proto Protobuf schemas, along with a binary file
+/// descriptor set for Protobuf schema reflection.
 fn main() -> Result<(), Box<dyn std::error::Error>> {
-    // Generates Rust code from .proto Protobuf schemas.
+    let out_dir = PathBuf::from(env::var("OUT_DIR")?);
     tonic_build::configure()
         .bytes(["."])
+        .file_descriptor_set_path(out_dir.join("page_api_descriptor.bin"))
         .compile_protos(&["proto/page_service.proto"], &["proto"])
         .map_err(|err| err.into())
 }

pageserver/page_api/proto/page_service.proto

@@ -11,6 +11,19 @@
 // - neon-shard-id: shard ID, as <number><count> in hex ("0b10" = shard 11 of 16, 0-based)
 // - neon-timeline-id: timeline ID ("f08c4e9a2d5f76b1e3a7c2d8910f4b3e")
 //
+// The service can be accessed via e.g. grpcurl:
+//
+//    ```
+//    grpcurl \
+//      -plaintext \
+//      -H "neon-tenant-id: 7c4a1f9e3bd6470c8f3e21a65bd2e980" \
+//      -H "neon-shard-id: 0b10" \
+//      -H "neon-timeline-id: f08c4e9a2d5f76b1e3a7c2d8910f4b3e" \
+//      -H "authorization: Bearer $JWT" \
+//      -d '{"read_lsn": {"request_lsn": 1234567890}, "rel": {"spc_oid": 1663, "db_oid": 1234, "rel_number": 5678, "fork_number": 0}}'
+//      localhost:51051 page_api.PageService/CheckRelExists
+//    ```
+//
 // TODO: consider adding neon-compute-mode ("primary", "static", "replica").
 // However, this will require reconnecting when changing modes.
 //
@@ -20,7 +33,7 @@
 // - Compression
 
 syntax = "proto3";
-package page_service;
+package page_api;
 
 service PageService {
   // Returns whether a relation exists.

pageserver/page_api/src/lib.rs

@@ -7,7 +7,12 @@
 
 // Code generated by protobuf.
 pub mod proto {
-    tonic::include_proto!("page_service");
+    tonic::include_proto!("page_api");
+
+    /// File descriptor set for Protobuf schema reflection. This allows using
+    /// e.g. grpcurl with the API.
+    pub const FILE_DESCRIPTOR_SET: &[u8] =
+        tonic::include_file_descriptor_set!("page_api_descriptor");
 
     pub use page_service_client::PageServiceClient;
     pub use page_service_server::{PageService, PageServiceServer};