feat: Allow configuration of additional headers

[mterhar]

Which problem is this PR solving?

If running Refinery in an environment that requires mTLS, we need to be able to configure a header to let the proxy know where to pass the traffic along.

Short description of the changes

Adds a configuration that injects headers into each of the signals that comes out of Refinery that are destined for Honeycomb.

1. Telemetry data flowing through the sampling proxy has it added to the DirectTransport http client
2. Logs from the Honeycomb logger have it added to the libhoney-go as a Round Tripper
3. Metrics coming from the OpenTelemetry metrics emitter have them added to the exporter
4. Experimental internal tracing has them added as well, just for troubleshooting if it's ever necessary

[mterhar]

Had to make a tweak to the markdown/yaml generator that uses the configMeta.yaml file. The example I made has 2 colons and since we don't use colon+space to delimit like yaml does, I had to make a splitter function that just looks for the first delimiter and stops trying to split afterwards.

[kentquirk]

A couple of thoughts; I'll leave to @robbkidd for final approval.

[robbkidd]

I tried this branch out sending locally. I gave it one of the reserved headers and …

❯ go run ./cmd/refinery --config ./config.yaml --rules_config rules.yaml
cannot override reserved Honeycomb header "X-Honeycomb-Team" in Network.AdditionalHeaders
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x108 pc=0x102c43be0]

goroutine 1 [running]:
github.com/honeycombio/refinery/logger.GetLoggerImplementation({0x0?, 0x0?})
        .../refinery/logger/logger.go:32 +0x20
main.main()
        .../refinery/cmd/refinery/main.go:114 +0x464
exit status 2

The automated tests don't exercise main where the panic happens. This has me realize that this config validation is happening entirely outside of our existing config validation process … which is ideally where it ought to occur.

[robbkidd]

I tried this branch out sending locally. [… and it panicked]

I can't reproduce this any more, so disregard!