Inconsistent debug logging of line payloads from TCP listener

[matthiasr]

We log raw packets in different places. Sometimes, we pass it to the logger as a string, sometimes as a byte slice. Specifically, this affects the TCP ingestion path.

This leads to confusing log output, especially in the JSON format, because the "binary" data is Base64-encoded by the log library.

We should consistently convert the lines to strings for logging, since we know that it's mostly-ASCII data.

[matthiasr]

In the standard log output, this shows as line="…" vs. line=…:

level=debug ts=2023-05-16T11:59:48.597Z caller=listener.go:136 msg="Incoming line" proto=tcp line=ratelimit.go.nextGC:43558224|g
level=debug ts=2023-05-16T11:59:48.597Z caller=relay.go:150 msg="Relaying line" line=ratelimit.go.nextGC:43558224|g
level=debug ts=2023-05-16T11:59:48.598Z caller=relay.go:119 msg="Adding line to buffer" line="ratelimit.go.nextGC:43558224|g\n"
level=debug ts=2023-05-16T11:59:49.569Z caller=relay.go:132 msg="Sending packet" length=597 data="ratelimit.go.numGC:1|c\nratelimit.go.totalAlloc:9974800|c\nratelimit.go.mallocs:131674|c\nratelimit.go.frees:312509|c\nratelimit.go.pauseTotalNs:99290|c\nratelimit.go.lastGC:1684238384274161217|g\nratelimit.go.gcCPUPercent:0|g\nratelimit.go.heapObjects:128909|g\nruntime.num_values:1|g\nratelimit.redis_pool.cx_active:10|g\nratelimit.go.sys:0|g\nratelimit.go.heapSys:51544064|g\nratelimit.go.heapIdle:20365312|g\nratelimit.go.nextGC:43558224|g\nratelimit.go.heapReleased:5029888|g\nratelimit.go.numGoroutine:464|g\nratelimit.go.alloc:27489712|g\nratelimit.go.heapAlloc:27489712|g\nratelimit.go.heapInuse:31178752|g\n"