Set User Agent Header for S3 Requests (mozilla#2137)

* Set User Agent Header for S3 Requests

Closes mozilla#2136.

This PR configures the S3 backend to use a custom HTTP client
which has a user agent header set.

The user agent header enables `sccache` users to write AWS policies to
accept or reject `sccache` requests to an S3 bucket based on their `sccache`
version.

Cargo.toml

@@ -164,7 +164,7 @@ memcached = ["opendal/services-memcached"]
 native-zlib = []
 oss = ["opendal/services-oss", "reqsign"]
 redis = ["url", "opendal/services-redis"]
-s3 = ["opendal/services-s3", "reqsign"]
+s3 = ["opendal/services-s3", "reqsign", "reqwest"]
 webdav = ["opendal/services-webdav"]
 # Enable features that will build a vendored version of openssl and
 # statically linked with it, instead of linking against the system-wide openssl

src/cache/s3.rs

@@ -11,8 +11,10 @@
 // limitations under the License.
 
 use opendal::layers::LoggingLayer;
+use opendal::raw::HttpClient;
 use opendal::services::S3;
 use opendal::Operator;
+use reqwest::ClientBuilder;
 
 use crate::errors::*;
 
@@ -29,6 +31,7 @@ impl S3Cache {
         server_side_encryption: Option<bool>,
     ) -> Result<Operator> {
         let mut builder = S3::default();
+        builder.http_client(set_user_agent());
         builder.bucket(bucket);
         builder.root(key_prefix);
 
@@ -63,6 +66,13 @@ impl S3Cache {
     }
 }
 
+/// Set the user agent (helps with monitoring on the server side)
+fn set_user_agent() -> HttpClient {
+    let user_agent = format!("{}/{}", env!("CARGO_PKG_NAME"), env!("CARGO_PKG_VERSION"));
+    let client_builder = ClientBuilder::new().user_agent(user_agent);
+    HttpClient::build(client_builder).unwrap()
+}
+
 /// Resolve given endpoint along with use_ssl settings.
 fn endpoint_resolver(endpoint: &str, use_ssl: Option<bool>) -> Result<String> {
     let endpoint_uri: http::Uri = endpoint