[internal] Export function to format user agent data so we can share it between non-HTTP and HTTP clients. (Azure#19681)

Export some code from `internal` so we that we can use the same user agent formatting code in both azservicebus+azeventhubs as we do in our HTTP clients.

Part of the work for Azure#19673

Author: richardpark-msft

sdk/internal/CHANGELOG.md

@@ -4,6 +4,8 @@
 
 ### Features Added
 
+- Export user agent formatting code that used to be in azcore's policy_telemetry.go so it can be shared with non-HTTP clients (ie: azservicebus/azeventhubs). ([#19681](https://github.com/Azure/azure-sdk-for-go/pull/19681))
+
 ### Breaking Changes
 
 ### Bugs Fixed

sdk/internal/telemetry/telemetry.go

@@ -0,0 +1,33 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT License.
+
+package telemetry
+
+import (
+	"fmt"
+	"os"
+	"runtime"
+)
+
+// Format creates the properly formatted SDK component for a User-Agent string.
+// Ex: azsdk-go-azservicebus/v1.0.0 (go1.19.3; linux)
+// comp - the package name for a component (ex: azservicebus)
+// ver - the version of the component (ex: v1.0.0)
+func Format(comp, ver string) string {
+	// ex: azsdk-go-azservicebus/v1.0.0 (go1.19.3; windows)
+	return fmt.Sprintf("azsdk-go-%s/%s %s", comp, ver, platformInfo)
+}
+
+// platformInfo is the Go version and OS, formatted properly for insertion
+// into a User-Agent string. (ex: '(go1.19.3; windows')
+// NOTE: the ONLY function that should write to this variable is this func
+var platformInfo = func() string {
+	operatingSystem := runtime.GOOS // Default OS string
+	switch operatingSystem {
+	case "windows":
+		operatingSystem = os.Getenv("OS") // Get more specific OS information
+	case "linux": // accept default OS info
+	case "freebsd": //  accept default OS info
+	}
+	return fmt.Sprintf("(%s; %s)", runtime.Version(), operatingSystem)
+}()

sdk/internal/telemetry/telemetry_test.go

@@ -0,0 +1,27 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT License.
+
+package telemetry
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestFormat(t *testing.T) {
+	userAgent := Format("azservicebus", "v1.0.0")
+
+	// Examples:
+	// * azsdk-go-azservicebus/v1.0.0 (go1.19.3; linux)
+	// * azsdk-go-azservicebus/v1.0.0 (go1.19; Windows_NT)
+	//
+	// The OS varies based on the actual platform but it's a small set.
+	re := `^azsdk-go-azservicebus/v1.0.0` +
+		` ` +
+		`\(` +
+		`go\d+\.\d+(|\.\d+); (Windows_NT|linux|freebsd)` +
+		`\)$`
+
+	require.Regexp(t, re, userAgent)
+}