Convert mongodb plugin to instrumentation

- Change folder name from `opentelemetry-plugin-*` to `opentelemetry-instrumentation-*`
- Change package name from `@opentelemetry/plugin-*` to `@opentelemetry/instrumentation-*`
- Change implementation to use [InstrumentationBase](https://github.com/open-telemetry/opentelemetry-js/blob/master/packages/opentelemetry-instrumentation/src/platform/node/instrumentation.ts#L30)