Fix tags concatenation in custom metric

[VitoMakarevich]

What does this PR do?

Fix concatenated tags for custom metrics.

Motivation

A bug that prevents from submitting more than 1 tag.
Bugfix for #75

Testing Guidelines

Unit test and test on a real AWS environment with the custom-built jar.

Types of changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

Checklist

• This PR's description is comprehensive
• This PR contains breaking changes that are documented in the description
• This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• This PR impacts documentation, and it has been updated (or a ticket has been logged)
• This PR's changes are covered by the automated tests
• This PR collects user input/sensitive content into Datadog

[VitoMakarevich]

@IvanTopolcic please take a look

[IvanTopolcic]

Looks awesome! Thanks so much for the fix and report, we really appreciate it! I'll go ahead and release a new version of the Java library and I'll close the issue once that's done.

[VitoMakarevich]

Thank you!