feat(tracing): Add instana header support in propagation

[SuzyWangIBMer]

Summary

Checklist

• The Pull Request has tests
• A changelog file has been created under changelog/unreleased/kong or skip-changelog label added on PR if changelog is unnecessary. README.md
• There is a user-facing docs PR against https://github.com/Kong/docs.konghq.com - PUT DOCS PR HERE

Issue reference

Fix #12494

[samugi]

Thank you so much for this great contribution! A couple of notes:

• For consistency, this should be supported by the Zipkin plugin as well, which requires a small schema update similar to what was done for OpenTelemetry in this PR, and an integration test in spec/03-plugins/34-zipkin/zipkin_spec.lua.
• The test in spec/01-unit/26-observability/02-propagation_strategies_spec.lua should be updated as well to include this new header format

[samugi]

In addition to my previous comment, this needs a rebase.

[SuzyWangIBMer]

I have updated trace_id, span_id, level_id variable name to trace_id_raw, span_id_raw and level_id_raw

Also since we dont use correlation_type and correlationId here, changed

level_id_raw = level_id_raw:match("^([0-1])$") 
              or level_id_raw:match("^([0-1]),correlationType=(.-);correlationId=(.*)")

to

  level_id_raw = level_id_raw:match("^([0-1])$") 
                or level_id_raw:match("^([0-1]).")

[samugi]

Left a few comments.

Please also address my comments from the previous review:

• For consistency, this should be supported by the Zipkin plugin as well, which requires a small schema update similar to what was done for OpenTelemetry in this PR, and an integration test in spec/03-plugins/34-zipkin/zipkin_spec.lua

• The test in spec/01-unit/26-observability/02-propagation_strategies_spec.lua should be updated as well to include this new header format

[samugi]

let's add a nil check:

Suggested change

	local trace_id = from_hex(trace_id_raw) or nil
	local span_id = from_hex(span_id_raw) or nil
	local trace_id = trace_id_raw and from_hex(trace_id_raw) or nil
	local span_id = span_id_raw and from_hex(span_id_raw) or nil

[samugi]

the value of headers["name"] can be a string or a table, depending on whether the same header was passed more than once in the request, see: https://github.com/openresty/lua-nginx-module?tab=readme-ov-file#ngxreqget_headers

We should handle the scenario where multiple headers are passed, for example by ignoring the header if passed more than once, or grabbing the first value.

[samugi]

could we just:

Suggested change

	level_id_raw = level_id_raw:match("^([0-1])$")
	or level_id_raw:match("^([0-1]).")
	level_id_raw = level_id_raw:sub(1, 1)

[samugi]

this must be type boolean, see here

Suggested change

	local should_sample = level_id_raw or "1"
	local should_sample = level_id_raw == "1"