feat(opentelemetry): add upstream client span - #13946

Description

APISIX currently propagates the gateway SERVER span directly to an upstream service. Without a CLIENT span for the proxy hop, APM service maps cannot represent the APISIX-to-upstream relationship correctly.

This change:

• creates an apisix.upstream CLIENT span for every upstream attempt;
• propagates the CLIENT span context, including across retries;
• records upstream address, port, HTTP status, and accurate attempt timing;
• marks failed attempts and upstream 4xx/5xx responses as errors;
• keeps the CLIENT span when ext-plugin-post-resp performs the outbound request and then exits early;
• avoids exporting a synthetic CLIENT span when a later plugin exits without contacting upstream; and
• documents the new OpenTelemetry variables in English and Chinese.

The tests cover span hierarchy and propagation, HTTP/2 request isolation, retry attempts, connection failures, early exits with and without a custom upstream dispatch, timing bounds, and error status handling.

Which issue(s) this PR fixes:

Fixes #13945

Tests

• t/plugin/opentelemetry6.t — 75 assertions passed
• OpenTelemetry plugin test suite (opentelemetry.t through opentelemetry6.t) — 226 assertions passed
• targeted Lua syntax, lj-releng, test-code-style, Markdown lint, and git diff --check checks passed
• full make lint: LuaCheck passed; lj-releng still reports pre-existing Unicode separator comments in unrelated service-discovery files

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the English and Chinese documentation
• I have verified that this change is backward compatible