chore(http sink): refactor to new style (vectordotdev#18200)

* First iteration, unit tests pass.

* Extract common Service functionality for HTTP based stream sinks.

* docs touchup

* Touch ups

* spell checker

* fix rust doc

* hopefully fix encoding regression

* Try item sized batching

* Refactor a bit

* cleanup

* cleanup

* doc clean up

* extract common service code for re use in other HTTP sinks

* feeback sw

* duplicate

* feedback ds

* feedback ds

* clippy

* fix write issue

* fix docs

docs/tutorials/sinks/2_http_sink.md

@@ -290,6 +290,12 @@ where the data is actually sent.
 
 # Service
 
+**⚠ NOTE! This section implements an HTTP tower `Service` from scratch, for the
+purpose of demonstration only. Many sinks will require implementing `Service`
+in this way. Any new HTTP-based sink should ideally utilize the
+`HttpService` structure, which abstracts away most of the logic shared
+amongst HTTP-based sinks.**
+
 We need to create a [`Tower`][tower] service that is responsible for actually
 sending our final encoded data.
 

src/sinks/appsignal/service.rs

@@ -24,6 +24,9 @@ use super::request_builder::AppsignalRequest;
 
 #[derive(Clone)]
 pub(super) struct AppsignalService {
+    // TODO: `HttpBatchService` has been deprecated for direct use in sinks.
+    //       This sink should undergo a refactor to utilize the `HttpService`
+    //       instead, which extracts much of the boilerplate code for `Service`.
     pub(super) batch_service:
         HttpBatchService<Ready<Result<http::Request<Bytes>, crate::Error>>, AppsignalRequest>,
 }

src/sinks/datadog/events/service.rs

@@ -43,6 +43,9 @@ impl DriverResponse for DatadogEventsResponse {
 
 #[derive(Clone)]
 pub struct DatadogEventsService {
+    // TODO: `HttpBatchService` has been deprecated for direct use in sinks.
+    //       This sink should undergo a refactor to utilize the `HttpService`
+    //       instead, which extracts much of the boilerplate code for `Service`.
     batch_http_service:
         HttpBatchService<Ready<Result<http::Request<Bytes>, crate::Error>>, DatadogEventsRequest>,
 }

src/sinks/elasticsearch/service.rs

@@ -68,6 +68,9 @@ impl MetaDescriptive for ElasticsearchRequest {
 
 #[derive(Clone)]
 pub struct ElasticsearchService {
+    // TODO: `HttpBatchService` has been deprecated for direct use in sinks.
+    //       This sink should undergo a refactor to utilize the `HttpService`
+    //       instead, which extracts much of the boilerplate code for `Service`.
     batch_service: HttpBatchService<
         BoxFuture<'static, Result<http::Request<Bytes>, crate::Error>>,
         ElasticsearchRequest,