auto-instrumentation-fetch replaces headers when req.headers input is of type Map

[rdeavila94]

What happened?

Steps to Reproduce

initialize fetch instrumentation

fetch('https://foobar.com', {headers: new Map().set('foo','bar')}

Expected Result

A request to foobar.com, with the header foo in addition to any trace propagation headers

Actual Result

The header foo is omitted, and only the trace propagation headers are sent

Additional Details

This is a result of this function. Since it is not handling a type of Map, it is resorting to extracting the keys as if the headers are an object literal, which it isn't.

It is important to note that a type of Map is not, according to the type definition of Request.Header, a valid type. However, it is fairly reasonable to expect that some folk may pass a Map type as their headers when using fetch, and in JavaScript will not result in any type errors/warnings. A Map type functionality-wise is valid, as it works as a type for headers when using fetch. The current code results in a bug where users who use fetch and have set their headers with a Map type have their headers entirely replaced by instrumentation-fetch when using it, where they were previously present without instrumentation-fetch.

OpenTelemetry Setup Code

// see PR

package.json

// see PR

Relevant log output

No response

[rdeavila94]

I've opened this PR to fix the issue