Save built-in JSON object when agent start to allow instrumenting JSON.stringify

**Is your feature request related to a problem? Please describe.**

My team is experimenting with APM and one of the important things we want to instrument is our calls to `JSON.stringify`. My first try was to override the `JSON.stringify()` function like this:

```typescript
const defaultStringify = JSON.stringify;
JSON.stringify = (...args: any): any => {
const span = startSpan('stringify', 'JSON');
try {
const result = defaultStringify.apply(null, args as any);
span.end('success');
return result;
} catch (error) {
span.end('failure');
throw error;
}
};
```
However when starting the application I get a `max call stack exceeded` error.
My hypothesis is that it comes from the fact that the agent uses `JSON.stringify()` in the `getContextFromRequest()` function.

**Describe the solution you'd like**

I would like to open a PR where I would save the functions of the `JSON` object when the agent starts so that we can override the object and the agent would keep using the default function.

**Describe alternatives you've considered**

For now I'm working around this issue by creating a dedicated module which exposes an instrumented function that we use instead of the built-in `JSON.stringify`

```typescript
export const stringify = (value: any, replacer?: any, space?: number): string => {
const span = apm.startSpan('JSON.stringify', 'JSON');
try {
const result = JSON.stringify(value, replacer, space);
span.end('success');
return result;
} catch (error) {
span.end('failure');
throw error;
}
};
```

And I'm coupling that with a custom eslint rule to make sure everyone in the team uses this function and not the default one.

**Additional context**

I am willing to create the PR on my side but I first wanted to check with you if that would be an acceptable solution.