Fix context propagation across goroutines

[edeNFed]

What's changed

1. Added 4 utility functions that should be used by instrumentors:

• get_consistent_key: This function provides a key that works across any version of Go. This key should be used in the eBPF map created by the instrumentor to record the end timestamp at the end of the probed function.
• start_tracking_span(void *contextContext, struct span_context *sc) - This function should be used in order to perform in-process context propagation, meaning passing context between different instrumentations in the same applications (see below test case for example).
• stop_tracking_span(struct span_context *sc) - This function marks the passed span context as no more relevant for context propagation. This function should usually be called when recording the end timestamp of a span.
• get_parent_span_context(void *ctx) - Use this function in order to get the parent span context if available. This function receives the address of context.Context object.

2. Modified existing instrumentors to use those functions when reading/writing to eBPF maps and to performing context propagation between instrumentors.

3. Modified gRPC (client) instrumentation to inject headers by probing loopyWriter.headerHandler instead of http2Client createHeaderFields: In Go >= 1.17, the return value is passed by registers and not by the stack. Currently, eBPF is not able to modify registers' values, only memory. Changing the uprobe to attach to loopyWriter.headerHandler allow us to add context-propagation headers even on Go versions above 1.17.

Validation

I used the following code as a test case:

func main() {
	http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
		con, err := grpc.Dial("grpc-server.default:50051", grpc.WithTransportCredentials(insecure.NewCredentials()))
		if err != nil {
			panic(err)
		}

		stopCh := make(chan struct{})
		go func() {
			healthClient := healthpb.NewHealthClient(con)
			resp, err := healthClient.Check(r.Context(), &healthpb.HealthCheckRequest{})
			if err != nil {
				panic(err)
			}

			fmt.Fprintf(w, "gRPC server status: %s", resp.Status)
			close(stopCh)
		}()

		<-stopCh
	})

	fmt.Println("HTTP server started on port 8080")
	err := http.ListenAndServe(":8080", nil)
	if err != nil {
		panic(err)
	}
}

Which produces the following trace (after the applied fix)

Tested on Go 1.15 and 1.20, both on arm64 and amd64

[edeNFed]

Blocked by: #82

[MrAlias]

Related to #86

[edeNFed]

@open-telemetry/go-instrumentation-approvers this is ready for review, thanks! 🙏

[MikeGoldsmith]

This looks really good, thanks @edeNFed.

I've left some comments and suggestions.

It's worth noting #91 conflicts with this. Whichever PR lands first, will require the other to be rebased.

[MikeGoldsmith]

@edeNFed is this ready for another round of review or have you more changes planned?

[edeNFed]

@open-telemetry/go-instrumentation-approvers This is ready for another review
Thank you 🙏

[MikeGoldsmith]

Looks great, thanks @edeNFed 👍🏻