Remove all shared internal code refs from zipkin exporter

[jack-berg]

Related to #7863.

Removes all shared internal code refs from opentelemetry-expoter-zipkin.

This facilitates the plan to stop publishing in mid 2026 while guaranteeing that old zipkin releases can continue to be used with more recent SDK releases without risk of runtime breakages.

Places where I had to make decisions:

• Exporter metric utils: Utilities spread across io.opentelemetry.exporter.internal.* and io.opentelemetry.sdk.internal.* which are involved in recording internal exporter telemetry. I copied all these classes and did treeshaking to remove code unused in the zipkin context. Also removed zipkin refs from original classes because its no longer used.
• ThrottlingLogger: From io.opentelemetry.sdk.internal.*. I copied these classes and did treeshaking to remov code unused in the zipkin context.
• io.opentelemetry.api.internal.Utils.checkArgument: Not worth copying - just duplicated the trivial logic.
• io.opentelemetry.api.internal.InstrumentationUtil: Utility for coordination instrumentation cycles from the java agent instrumenting the client libraries used by the exporters. Can't make a copy of this. Need a single reliable ref to the ContextKey to share between SDK and agent. To address, I promoted to public impl api at io.opentelemetry.api.impl. @open-telemetry/java-instrumentation-maintainers we'll need to coordinate with a corresponding change in opentelemetry-java-instrumentation after this release.

[otelbot]

⚠️ API changes detected — additional maintainer review required

@jack-berg @jkwatson

This PR modifies the public API surface area of the following module(s):

• opentelemetry-api

Please review the changes in docs/apidiffs/current_vs_latest/ carefully before approving.

[jack-berg]

This is the key line that verifies I covered everything. If I missed anything, this test would fail.

[codecov]

Codecov Report

❌ Patch coverage is 80.06873% with 58 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.00%. Comparing base (d39116e) to head (87d95e3).

Files with missing lines	Patch %	Lines
...try/exporter/zipkin/internal/ThrottlingLogger.java	56.41%	14 Missing and 3 partials ⚠️
...orter/zipkin/internal/ExporterInstrumentation.java	71.05%	6 Missing and 5 partials ⚠️
...porter/zipkin/internal/SemConvExporterMetrics.java	90.58%	1 Missing and 7 partials ⚠️
...xporter/zipkin/internal/LegacyExporterMetrics.java	86.36%	2 Missing and 4 partials ⚠️
...etry/exporter/zipkin/internal/ExporterMetrics.java	71.42%	2 Missing and 2 partials ⚠️
.../exporter/zipkin/internal/NoopExporterMetrics.java	0.00%	4 Missing ⚠️
...elemetry/exporter/zipkin/internal/RateLimiter.java	76.47%	1 Missing and 3 partials ⚠️
...try/exporter/zipkin/ZipkinSpanExporterBuilder.java	50.00%	1 Missing and 1 partial ⚠️
...elemetry/exporter/zipkin/internal/ComponentId.java	87.50%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #8413      +/-   ##
============================================
- Coverage     91.15%   91.00%   -0.15%     
- Complexity     7762     7811      +49     
============================================
  Files           881      892      +11     
  Lines         23408    23691     +283     
  Branches       2330     2360      +30     
============================================
+ Hits          21337    21561     +224     
- Misses         1377     1409      +32     
- Partials        694      721      +27     

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