Switch to using per header field parsing

[grcevski]

This PR is changing how we parse handle the Go HTTP request headers to switch from parsing the Go map to using per header uprobe.

The existing tests should be sufficient to prove this initial version works, since we are already testing for context propagation.

In this first version of the PR I've simply switched how the parsing is done to use the new approach. This approach works regardless of the Go version used, e.g. it will work for Go < 1.24 and Go >= 1.24.

I'm looking for feedback on if we should add a load time constant for the BPF program for the Go version. If we test for the Go version in the BPF code we can choose to use the old approach and not mount the new probe.

There are few pros and cons to each approach:

Proposal 1: We don't read any Go maps until we have swiss maps support

Pros:

• Simpler to test, since all of our tests will be exercising the same code regardless of the Go version used.
• Code is much simpler to reason about
• Go iterates quickly on releases and old versions are quickly dropped out of support

Cons:

• All go versions instrumentations are slower even for those that we know to read the maps for.

Proposal 2: We use the Go version to turn this on and off

Pros:

• Older Go version instrumentations will be faster
• If we ever implement support for swiss maps most of the code will already be there to handle < 1.24 and >= 1.24

Cons:

• We need to add Go 1.24 release candidate tests to prove this new code works
• Potential bugs might be version dependent
• Diminishing returns for the complexity with new Go releases, unless we implement swiss map support quickly.

Relates to #1318

[grcevski]

I added a helper we have used before in Beyla which can do case insensitive compares for the header fields. e.g. Traceparent, TRACEPARENT, TraceParent, traceparent all match.

[RonFed]

Can you please add the function signature as a comment?

[RonFed]

We have maps used for context tracking in:

opentelemetry-go-instrumentation/internal/include/go_context.h

Lines 14 to 21 in 5f77fdb

	struct
	{
	__uint(type, BPF_MAP_TYPE_HASH);
	__type(key, void *);
	__type(value, struct span_context);
	__uint(max_entries, MAX_CONCURRENT_SPANS);
	__uint(pinning, LIBBPF_PIN_BY_NAME);
	} go_context_to_sc SEC(".maps");

To use these we need:

• pass NULL to get_parent_span_context_fn which will default to use get_parent_span_context.
• Call start_tracking_span to update the maps.
• Call stop_tracking_span to delete the entries.

[RonFed]

We can use the package constraint we have for the go versions as defined in:

opentelemetry-go-instrumentation/internal/pkg/instrumentation/bpf/go.opentelemetry.io/otel/traceglobal/probe.go

Line 60 in 5f77fdb

goWithoutSwissMaps = probe.PackageConstrainst{

This will only load the probe for version which have Swiss maps.
It also means will have to inject a constant specifying whether to extract the headers the old or the new way - I think that's a good solution until we'll have full support for Swiss maps.