Java OtlpHttpMetricExporter does not produce JSON formatted output

[prashanthr-sn]

Describe the bug
Per the docs, setting environment variable OTEL_EXPORTER_OTLP_METRICS_PROTOCOL or setting Java system property to "http/json" is supposed to produce JSON formatted output. However, neither of the settings produce JSON.

https://opentelemetry.io/docs/concepts/sdk-configuration/otlp-exporter-configuration/#otel_exporter_otlp_metrics_protocol

https://github.com/open-telemetry/opentelemetry-java/blob/main/sdk-extensions/autoconfigure/README.md#otlp-exporter-span-metric-and-log-exporters

Steps to reproduce
Any instrumented program using OtlpHttpMetricExporter.builder()....build() and run with the environment variable or system property set as described. Upon running the output is still binary/protobuf and not JSON. The Content-Type remains application/x-protobuf

What did you expect to see?
JSON formatted output. The Content-Type from the client should be "application/json" (https://opentelemetry.io/docs/specs/otlp/#json-protobuf-encoding)

What did you see instead?
Output was binary/protobuf format and not JSON. The Content-Type remains application/x-protobuf

What version and what artifacts are you using?
Artifacts: (e.g., opentelemetry-api, opentelemetry-sdk, which exporters, etc)
Version: (e.g., v0.4.0, 1eb551b, etc)
How did you reference these artifacts? (excerpt from your build.gradle, pom.xml, etc)

Environment
Compiler: Azul JDK 8
OS: Mac OS
Runtime (if different from JDK above): (e.g., "Oracle JRE 8u251")
OS (if different from OS compiled on): (e.g., "Windows Server 2019")

Additional context
Looking at

opentelemetry-java/exporters/otlp/all/src/main/java/io/opentelemetry/exporter/otlp/http/metrics/OtlpHttpMetricExporterBuilder.java

Line 160 in f20ee6c

OtlpHttpMetricExporterBuilder exportAsJson() {

there is an exportAsJson(). But, the method is not public. Is it supposed to have been public?

The test class

opentelemetry-java/exporters/otlp/all/src/test/java/io/opentelemetry/exporter/otlp/internal/OtlpMetricExporterProviderTest.java

Line 4 in f20ee6c

*/

also does not have any tests for "http/json", but only for "http/protobuf".
So.. is JSON output supported?

[mateuszrzeszutek]

Hey @prashanthr-sn ,

SDK support for the http/json format is optional:

SDKs SHOULD support both grpc and http/protobuf transports and MUST support at least one of them. If they support only one, it SHOULD be http/protobuf. They also MAY support http/json.

The Java SDK does not have the JSON exporter implemented. Here's some more history about that: #3651 #3486
I'm quite convinced we're not planning to implement it in the future -- ping @jack-berg to confirm.

[jack-berg]

I'm quite convinced we're not planning to implement it in the future

I can confirm that it's not supported and we do not plan on implementing it. Closing this issue accordingly.