perf: fewer memory allocation

[joeyzhao2018]

What does this PR do?

Less memory allocation and better performance.

Explanation

Memory allocation timing:

• d.get("k", {}) always creates a new empty dictionary when the function is called, regardless of whether it will be used
• d.get("k") or {} only creates the empty dictionary if d.get("k") returns a falsy value

Handling of falsy values:

• d.get("k", {}) only uses the default when the key is missing
• d.get("k") or {} uses the empty dict when the key is missing OR when the value exists but is falsy (None, empty string, 0, False, etc.)

Performance implications:

• d.get("k", {}) has a slight overhead of always creating an object that might be discarded
• d.get("k") or {} is slightly more efficient when the key exists with a truthy value

this is a follow-up PR for PR #593.
https://datadoghq.atlassian.net/browse/APMSVLS-10

Motivation

Testing Guidelines

Additional Notes

Types of Changes

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

Check all that apply

• 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
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• This PR passes the integration tests (ask a Datadog member to run the tests)