feat(context): add setting sampled flag in create

Author: Stephen Belanger

lib/instrumentation/context.js

@@ -24,14 +24,22 @@ class TraceContext {
     this.parentId = parentId
   }
 
-  static create () {
+  static create (sampled = false) {
     const bytes = crypto.randomBytes(24)
+    const flags = Buffer.alloc(1)
+
+    // Load sampled flag into first bit of flags
+    if (sampled) {
+      flags[0] |= 0x01
+    } else {
+      flags[0] &= ~0x01
+    }
 
     return new TraceContext({
       version: Buffer.alloc(1),
       traceId: bytes.slice(0, 16),
       id: bytes.slice(16),
-      flags: Buffer.alloc(1)
+      flags: flags
     })
   }