Environment details

• OS: Mac OS
• Node.js version: 14.15.4
• npm version: 7.21.1
• @google-cloud/trace-agent version: 5.1.5

Steps to reproduce

1. import TraceAgent from '@google-cloud/trace-agent'
2. Run TraceAgent.start();
3. Observe the error TypeError: Cannot read property 'start' of undefined

To fix this, I needed to instead import * as TraceAgent

Suggestions/Questions:

1. Can this be added to the readme, to help with future integrators using es6/typescript imports?
2. I wonder why this wasn't caught by typescript? is there something wrong with the typing provided?