Use RESOURCE_NAME constant for resource tag check

[montmanu]

What does this PR do?

Replaces hard-coded value for tag key used to identify a span's resource name with a reference to the corresponding constant.

Motivation

I was reviewing the source code to determine how a particular resource name was being assigned when I noticed an inconsistency between this implementation (the hard-coded string) vs others within the same package (which reference the RESOURCE_NAME instead).

For example, the wrapMiddleware function sources the tag key name from the constant:

dd-trace-js/packages/dd-trace/src/plugins/util/web.js

Lines 192 to 194 in 68516f7

	span.addTags({
	[RESOURCE_NAME]: middleware._name || middleware.name || '<anonymous>'
	})

Even within the addResourceTag function's implementation, there seems to be an inconsistency between the following:

dd-trace-js/packages/dd-trace/src/plugins/util/web.js

Line 520 in 68516f7

if (tags['resource.name']) return

vs

dd-trace-js/packages/dd-trace/src/plugins/util/web.js

Line 526 in 68516f7

span.setTag(RESOURCE_NAME, resource)

Additional Notes

Here is where the RESOURCE_NAME const is defined:

dd-trace-js/ext/tags.js

Line 6 in 68516f7

RESOURCE_NAME: 'resource.name',