[samplers/jaegerremote] Invalid expectation of JSON format

[yurishkuro]

Background

Originated from open-telemetry/opentelemetry-specification#3126

The official Jaeger remote sampling protocol returns strategyType enums as strings. Both jaeger_agent:5778/sampling and jaeger_collector:14268/api/sampling endpoints return that format. OTel Collector also returns the same format (code, unit test). There is another, long-deprecated, format with enums as numbers, which is historically returned from jaeger_agent:5778/ endpoint (only very early versions of Jaeger SDKs have used this in 2015, before Thrift 0.9.3 was released):

# official
$ curl "http://localhost:5778/sampling?service=foo"
{"strategyType":"PROBABILISTIC","probabilisticSampling":{"samplingRate":1}}

# deprecated, legacy
$ curl "http://localhost:5778/?service=foo"
{"strategyType":0,"probabilisticSampling":{"samplingRate":1}}

Problem

This extension expects the legacy format with enums as numbers, as demonstrated by this PR: #3183. Thus the SDK extension is completely incompatible with the OTel Collector's extension jaegerremotesampling, and is only compatible with the legacy endpoint in Jaeger Agent.

Proposed Solution

• Option 1: fix this extension to expect the official format (potentially breaking change for existing deployments)
• Option 2: add a configuration option
• Option 3: sniff the payload and recognize both formats (e.g., the way Jaeger generates the legacy format is by first rendering the official format and then doing a dump string substitution to replace string enums with numbers). I would go the other way in this change, numbers to strings, so that official gRPC JSON parsing could be used.

[yurishkuro]

cc @tttoad @vuuihc @kvrhdn - any thoughts?

[yurishkuro]

I went with a variant of Option 3 by using gogoproto/jsonpb parser that understands both enum formats: #3183