Add `cmd-exec-id` to user agent (#1808)

## Changes

This PR adds the `cmd-exec-id` field to the user agent. This allows us
to correlate multiple HTTP requests made from the CLI.

### Why Not Use HTTP traceparent?
We considered using the traceparent header in HTTP as an alternative,
but it's not a good fit for our use case. Here's why:
1. Purpose of traceparent: It's designed to trace a single HTTP request
across a distributed system as it moves through subsystems and proxies.
2. Our requirement: We need to trace multiple HTTP requests made during
a single command execution in the CLI.

For more details about how traceparent itself works and how it's used in
the Go SDK, see
https://github.com/databricks/databricks-sdk-go/pull/914.

## Tests
Unit test

cmd/root/root.go

@@ -75,6 +75,7 @@ func New(ctx context.Context) *cobra.Command {
 
 		// Configure our user agent with the command that's about to be executed.
 		ctx = withCommandInUserAgent(ctx, cmd)
+		ctx = withCommandExecIdInUserAgent(ctx)
 		ctx = withUpstreamInUserAgent(ctx)
 		cmd.SetContext(ctx)
 		return nil

cmd/root/user_agent_command_exec_id.go

@@ -0,0 +1,14 @@
+package root
+
+import (
+	"context"
+
+	"github.com/databricks/databricks-sdk-go/useragent"
+	"github.com/google/uuid"
+)
+
+func withCommandExecIdInUserAgent(ctx context.Context) context.Context {
+	// A UUID that will allow us to correlate multiple API requests made by
+	// the same CLI invocation.
+	return useragent.InContext(ctx, "cmd-exec-id", uuid.New().String())
+}

cmd/root/user_agent_command_exec_id_test.go

@@ -0,0 +1,26 @@
+package root
+
+import (
+	"context"
+	"regexp"
+	"testing"
+
+	"github.com/databricks/databricks-sdk-go/useragent"
+	"github.com/google/uuid"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestWithCommandExecIdInUserAgent(t *testing.T) {
+	ctx := withCommandExecIdInUserAgent(context.Background())
+
+	// Check that the command exec ID is in the user agent string.
+	ua := useragent.FromContext(ctx)
+	re := regexp.MustCompile(`cmd-exec-id/([a-f0-9-]+)`)
+	matches := re.FindAllStringSubmatch(ua, -1)
+
+	// Assert that we have exactly one match and that it's a valid UUID.
+	require.Len(t, matches, 1)
+	_, err := uuid.Parse(matches[0][1])
+	assert.NoError(t, err)
+}

cmd/root/user_agent_command_test.go

@@ -1,13 +1,15 @@
 package root
 
 import (
+	"context"
 	"testing"
 
+	"github.com/databricks/databricks-sdk-go/useragent"
 	"github.com/spf13/cobra"
 	"github.com/stretchr/testify/assert"
 )
 
-func TestCommandString(t *testing.T) {
+func TestWithCommandInUserAgent(t *testing.T) {
 	root := &cobra.Command{
 		Use: "root",
 	}
@@ -26,4 +28,9 @@ func TestCommandString(t *testing.T) {
 	assert.Equal(t, "root", commandString(root))
 	assert.Equal(t, "hello", commandString(hello))
 	assert.Equal(t, "hello_world", commandString(world))
+
+	ctx := withCommandInUserAgent(context.Background(), world)
+
+	ua := useragent.FromContext(ctx)
+	assert.Contains(t, ua, "cmd/hello_world")
 }