feat: Log and add agent, kernel and btfEnabled to events (#52)

## Which problem is this PR solving?

Adds agent, kernel and btf enabled fields to events.

- Closes #47 

## Short description of the changes
- Add BtfEnabled func to utils
- Log agent version, kernel version and if btf is enabled at startup
- Add agent version, kernel version and btfEnabled fields to all events

## How to verify that this has the expected result
- Agent logs agent version, kernel version and if btf is enabled at
start up
- Events sent to Honeycomb include fields for agent version, kernel
version and if btf is enabled

```sh
# Local Docker Desktop:
2023-08-03 10:15:00 2023/08/03 14:15:00 Starting Honeycomb eBPF agent v0.0.2
2023-08-03 10:15:00 2023/08/03 14:15:00 Host kernel version: 5.15.49
2023-08-03 10:15:00 2023/08/03 14:15:00 BTF enabled: false

# EKS on ARM
2023/08/03 14:19:26 Starting Honeycomb eBPF agent v0.0.2
2023/08/03 14:19:26 Host kernel version: 5.10.184
2023/08/03 14:19:26 BTF enabled: true
```


![event-with-metadata-eks-docker](https://github.com/honeycombio/honeycomb-ebpf-agent/assets/29520003/c2083f6c-411e-4a79-98b4-2bfc419af610)

---------

Co-authored-by: Mike Goldsmth <goldsmith.mike@gmail.com>

main.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"fmt"
 	"log"
 	"os"
 
@@ -9,20 +10,22 @@ import (
 	"github.com/honeycombio/libhoney-go"
 )
 
-const Version string = "0.0.1"
+const Version string = "0.0.2"
 const defaultDataset = "hny-ebpf-agent"
 const defaultEndpoint = "https://api.honeycomb.io"
 
 func main() {
 	log.Printf("Starting Honeycomb eBPF agent v%s\n", Version)
 
-	// Try to detect host kernel kernelVersion
 	kernelVersion, err := utils.HostKernelVersion()
 	if err != nil {
 		log.Fatalf("Failed to get host kernel version: %v", err)
 	}
 	log.Printf("Host kernel version: %s\n", kernelVersion)
 
+	btfEnabled := utils.HostBtfEnabled()
+	log.Printf("BTF enabled: %v\n", btfEnabled)
+
 	apikey := os.Getenv("HONEYCOMB_API_KEY")
 	if apikey == "" {
 		log.Fatalf("Honeycomb API key not set, unable to send events\n")
@@ -40,6 +43,15 @@ func main() {
 		Dataset: dataset,
 		APIHost: endpoint,
 	})
+
+	// appends libhoney's user-agent (TODO: doesn't work, no useragent right now)
+	libhoney.UserAgentAddition = fmt.Sprintf("hny/ebpf-agent/%s", Version)
+
+	// configure global fields that are set on all events
+	libhoney.AddField("honeycomb.agent_version", Version)
+	libhoney.AddField("meta.kernel_version", kernelVersion.String())
+	libhoney.AddField("meta.btf_enabled", btfEnabled)
+
 	defer libhoney.Close()
 
 	// setup probes

utils/utils.go

@@ -2,6 +2,7 @@ package utils
 
 import (
 	"fmt"
+	"os"
 
 	"github.com/cilium/ebpf/features"
 )
@@ -31,3 +32,8 @@ func HostKernelVersion() (KernelVersion, error) {
 	}
 	return KernelVersion(code), nil
 }
+
+func HostBtfEnabled() bool {
+	_, err := os.Stat("/sys/kernel/btf/vmlinux")
+	return err == nil
+}