apmbench: add protocol to loadgen config

[reakaleek]

Context

The MIS benchmarks are failing with:

apmbench-648… │ panic: invalid or unsupported protocol ()
apmbench-648… │ 
apmbench-648… │ goroutine 5 [running]:
apmbench-648… │ main.newEventHandler({0x0?, 0x0?}, {0xbec9d5, 0x8}, 0xc00050c050)
apmbench-648… │ 	/opt/apm-perf/cmd/apmbench/bench.go:215 +0x2c6
apmbench-648… │ main.benchmarkAgent(0x0?, 0x0?, {0xbec9d5?, 0x0?})
apmbench-648… │ 	/opt/apm-perf/cmd/apmbench/bench.go:221 +0x34
apmbench-648… │ main.BenchmarkAgentAll(0x0?, 0x0?)
apmbench-648… │ 	/opt/apm-perf/cmd/apmbench/bench.go:74 +0x25
apmbench-648… │ main.runOne.func1.1()
apmbench-648… │ 	/opt/apm-perf/cmd/apmbench/run.go:123 +0x67
apmbench-648… │ created by main.runOne.func1
apmbench-648… │ 	/opt/apm-perf/cmd/apmbench/run.go:121 +0xdb
apmbench-648… │      ┊ Scheduled       - <1s
apmbench-648… │      ┊ Initialized     - <1s
apmbench-648… │      ┊ Ready           - (…) Pending

in https://github.com/elastic/apm-managed-service/actions/runs/7858017783/job/21442506285

Details

This change sets apm/http as the default in the loadgen config.

Workflow run

Verified to work in https://github.com/elastic/apm-managed-service/actions/runs/7862337046/job/21451692616

[carsonip]

I believe some other parts of the code were broken by #46

e.g.

func BenchmarkAgentAll(b *testing.B, l *rate.Limiter) {
	benchmarkAgent(b, l, `*.ndjson`)
}

func BenchmarkAgentGo(b *testing.B, l *rate.Limiter) {
	benchmarkAgent(b, l, `go*.ndjson`)
}

func BenchmarkAgentNodeJS(b *testing.B, l *rate.Limiter) {
	benchmarkAgent(b, l, `nodejs*.ndjson`)
}

func BenchmarkAgentPython(b *testing.B, l *rate.Limiter) {
	benchmarkAgent(b, l, `python*.ndjson`)
}

func BenchmarkAgentRuby(b *testing.B, l *rate.Limiter) {
	benchmarkAgent(b, l, `ruby*.ndjson`)
}

but the ndjson are renamed

[carsonip]

I've proposed an alternative fix in #54. Let me know if it looks good.

[reakaleek]

I've proposed an alternative fix in #54. Let me know if it looks good.

Thanks!