added protobuf version as exporter user-agent version (#444)

Author: bryce-b

Sources/Exporters/OpenTelemetryProtocolCommon/common/Constants.swift

@@ -6,7 +6,10 @@
 import Foundation
 
 public enum Constants {
-    public enum HTTP {
-        public static let userAgent = "User-Agent"
-    }
+  public enum OTLP {
+    public static let version = "0.20.0"
+  }
+  public enum HTTP {
+    public static let userAgent = "User-Agent"
+  }
 }

Sources/Exporters/OpenTelemetryProtocolCommon/common/Headers.swift

@@ -7,15 +7,15 @@ import Foundation
 import OpenTelemetryApi
 
 public struct Headers {
-    // GetUserAgentHeader returns an OTLP header value of the form "OTel OTLP Exporter Swift/{{ .Version }}"
-    // https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/protocol/exporter.md#user-agent
-    public static func getUserAgentHeader() -> String {
-        var version = OpenTelemetry.version
-        if !version.isEmpty && version.hasPrefix("v") {
-            version = String(version.dropFirst(1))
-        }
-        let userAgent = "OTel-OTLP-Exporter-Swift/\(version)"
-
-        return userAgent
+  // GetUserAgentHeader returns an OTLP header value of the form "OTel OTLP Exporter Swift/{{ .Version }}"
+  // https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/protocol/exporter.md#user-agent
+  public static func getUserAgentHeader() -> String {
+    var version = Constants.OTLP.version
+    if !version.isEmpty && version.hasPrefix("v") {
+      version = String(version.dropFirst(1))
     }
+    let userAgent = "OTel-OTLP-Exporter-Swift/\(version)"
+    
+    return userAgent
+  }
 }