Decreases switch statements and deprecates BytesMessageEncoder

[codefromthecrypt]

I noticed in our senders and reporters there were multiple internal switch statements to get functions for a specific encoding. This is worse externally for zipkin-reporter-brave, as call sites need to duplicate switch statements, and also update them in order to allow another encoding. This is the case with PROTO3, for example, which is not yet supported for brave spans.

Here are the main changes:

• Add Encoding.encode(List<byte[]>) from the now deprecated BytesMessageEncoder.
• Add Encoding.mediaType(), useful in internal and external HTTP senders.
• Re-use the existing zipkin SpanBytesEncoder.forEncoding(Encoding) to reduce redundant switch logic.
• Add MutableSpanBytesEncoder.forEncoding(Encoding) for Brave
• Add MutableSpanBytesEncoder.create(Encoding, Tag<Throwable>) for Brave users who want an alt error tag.

With these changes in place, external senders such as those defined in spring-boot, can inherit sensible defaults without maintenance.

This acknowledges some specialized senders (such as okhttp) won't use Encoding.encode(List<byte[]>). However, this change makes commodity senders a lot easier to write and configure, as they can now be driven by just Encoding and an endpoint.

[codefromthecrypt]

seems like spring boot need to release soon, so I will check on this now. hopefully, looks ok to the team, but if you have a chance to take a look, please do!

[codefromthecrypt]

Here are some notable diffs.

-       HttpSender() {
-               super(Encoding.JSON);
+       HttpSender(Encoding encoding) {
+               super(encoding);
        }

        @Override
@@ -74,7 +74,7 @@ abstract class HttpSender extends BytesMessageSender.Base {
                if (this.closed) {
                        throw new ClosedSenderException();
                }
-               byte[] body = BytesMessageEncoder.JSON.encode(encodedSpans);
+               byte[] body = this.encoding.encode(encodedSpans);
                HttpHeaders headers = getDefaultHeaders();
                if (needsCompression(body)) {
                        body = compress(body);
@@ -86,7 +86,7 @@ abstract class HttpSender extends BytesMessageSender.Base {
        HttpHeaders getDefaultHeaders() {
                HttpHeaders headers = new HttpHeaders();
                headers.set("b3", "0");
-               headers.set("Content-Type", "application/json");
+               headers.set("Content-Type", this.encoding.mediaType());
                return headers;
        }

                @Bean
                @ConditionalOnMissingBean(value = MutableSpan.class, parameterizedContainer = BytesEncoder.class)
                @ConditionalOnEnabledTracing
-               BytesEncoder<MutableSpan> braveSpanEncoder() {
-                       return new JsonV2Encoder();
+               BytesEncoder<MutableSpan> braveSpanEncoder(Encoding encoding) {
+                       return MutableSpanBytesEncoder.forEncoding(encoding);
                }

                @Bean
@@ -157,8 +158,8 @@ class ZipkinConfigurations {
                @Bean
                @ConditionalOnMissingBean(value = Span.class, parameterizedContainer = BytesEncoder.class)
                @ConditionalOnEnabledTracing
-               BytesEncoder<Span> zipkinSpanEncoder() {
-                       return SpanBytesEncoder.JSON_V2;
+               BytesEncoder<Span> zipkinSpanEncoder(Encoding encoding) {
+                       return SpanBytesEncoder.forEncoding(encoding);
                }

[codefromthecrypt]

thanks for the look @anuraaga @reta! I need to add the long overdue brave proto encoder next to avoid folks running into a wall. It is a somewhat mechanical port of what's in zipkin and brave already..