Build the Via request and response headers with strconv-based
concatenation instead of fmt.Sprintf, avoiding fmt's reflection and
formatting overhead on every proxied request/response. Same single
allocation, ~40% faster for that construction in isolation.
| before (fmt) | after (strconv) |
| sec/op | sec/op vs base |
PrepareRequest-8 | 6.136µ ± 11% | 4.491µ ± 33% -26.80% (p=0.007 n=10)|
B/op: 1008 -> 1008 (unchanged)
allocs/op: 12 -> 12 (unchanged)
No behavior change. Adds BenchmarkPrepareRequest, as per policy, exercising the
request preparation path.
