upgrade thanos from v0.39.2 to v0.41.0,trace-id loss.

[zdyj3170101136]

in v0.39.2, the thanos query frontend slow log would print trace-id。

but after upgraded to v0.41.0, the trace-id is lossed。

v0.39.2 extract trace-id code:

	hs := w.Header()
	for h, vs := range resp.Header {
		hs[h] = vs
	}
	// ...
	if shouldReportSlowQuery {
		f.reportSlowQuery(r, hs, queryString, queryResponseTime, stats)
	}

v0.41.0 extrace trace-id code:

		if shouldReportSlowQuery {
			var responseHeaders http.Header
			if resp != nil {
				responseHeaders = resp.Header
			}
			f.reportSlowQuery(r, responseHeaders, queryString, queryResponseTime, stats)
		}

the header is different, i think this is why we lose the trace-id。

[zdyj3170101136]

i think is this mr import this problem.
#8618
@pvlltvk cc

[pvlltvk]

@zdyj3170101136
Hi!
Indeed, #8618 is the reason. I also bumped into this issue in my own environment and going to provide the PR with the fix soon

[GiedriusS]

@zdyj3170101136 could you help with reviewing the above PR?

[zdyj3170101136]

@zdyj3170101136您能否帮忙审核一下上面的公关稿？

i test the mr in prod, the problem is fixed, lgtm.