fix(datadog): use the username property instead of the whole table when generating the tag

[ecoupal-believe]

Description

Use the username property instead of the whole table when generating the tag.

Fixes https://github.com/apache/apisix/issues/9269

Checklist

• [x] I have explained the need for this PR and the problem it solves
• [x] I have explained the changes or the new features added to this PR
• [x] I have added tests corresponding to this change
• [x] I have updated the documentation to reflect this change
• [x] I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)

[shreemaan-abhishek]

@ecoupal-believe please add test cases as well.

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[bzp2010]

Fixed by #11354, thanks @ecoupal-believe 🚀