fix(opentelemetry): preserve booleans, handle multi-value headers, tighten test

[shreemaan-abhishek]

Description

Three follow-ups to #13146 (which started coercing additional_attributes values to strings via tostring() before passing them to attr.string()). The patch as merged still has three discrete gaps:

1. Boolean false is silently dropped

inject_attributes() gates insertion on if val then. Lua treats false as falsy, so any additional_attribute whose resolved value is the boolean false (from an nginx variable, or set explicitly by a plugin) never makes it into the span at all. Switching to if val ~= nil then is the standard Lua idiom for "exists, including false".

2. Multi-value headers stringify to "table: 0x..."

When additional_attributes references a header source via prefix-match (X-Foo-*), the value comes from core.request.headers(api_ctx) which wraps ngx.req.get_headers(). That can return a Lua table for multi-value headers (multiple Set-Cookie, multiple X-Forwarded-For, etc.). tostring(some_table) then emits "table: 0x7f..." and that's what ends up as the OTel attribute value — useless.

This PR extracts a coerce_attr_value(val) helper that:

• Returns nil for nil (caller skips the attribute).
• Joins table values with ", " (table.concat).
• Falls through to tostring() for everything else.

Both the prefix-match path and the keyed path go through the helper.

3. TEST 22 is too loose to detect a regression

The new TEST 22's response_body assertion is:

qr/.*opentelemetry-lua.*/

That's the OTel exporter's library-name marker; every OTel export contains it, regardless of whether additional_attributes actually made it into the span. The test would still pass with the regression #13146 was supposed to fix.

Tighten it to assert each of request_time and bytes_sent appears as a "stringValue" in the export:

qr/.*opentelemetry-lua.*"key":"request_time","value":\{"stringValue":"[^"]+"\}.*"key":"bytes_sent","value":\{"stringValue":"[^"]+"\}.*/s

upstream_response_time is intentionally not asserted: TEST 21 calls /opentracing which is served directly without proxy_pass, so $upstream_response_time is nil and the plugin correctly omits the attribute. Asserting it would fail for the wrong reason.

Which issue(s) this PR fixes:

Follow-up to #13146; no separate issue.

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 — TEST 22 in t/plugin/opentelemetry.t is now a real regression guard for the string-coercion contract.
• I have updated the documentation to reflect this change — no doc impact; the plugin's documented contract is unchanged.
• I have verified that this change is backward compatible