fix fetch instrumentation not handling headers array correctly

[t2t2]

Description

Other than object ({'X-Test': 'value'}) and Headers object, there's a third way to pass headers into fetch api of [name, value][] ([['X-Test', 'value']]) (spec). This currently isn't handled correctly in otel's fetch instrumentation, ending up converting the array into object {0: 'X-Test,value'}

Bypassing the bug for now by converting the array into a Headers object

Type of change

• Bug fix (non-breaking change which fixes an issue)

How has this been tested?

• Manual testing

fetch('/some-data', {headers: [['X-Test', 'value']]}).then(response => response.json()).then(console.log);