Only replace the logging formatter if it is a logging.Formatter

[agocs]

What does this PR do?

Makes the trace context log injection code check the type of the logging Formatter before replacing the Formatter. If the user is using a JSON formatter, it will pick up the trace context from the dummy span that contains the trace context.

Motivation

#153

Testing Guidelines

Screenshots of it working:

(JSON logging, no lambda library)

(JSON logging, old version of the lambda library)

(JSON logging, new version of the lambda library)

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
• This PR collects user input/sensitive content into Datadog
• This PR passes the integration tests (ask a Datadog member to run the tests)

[nhinsch]

If the user is using a JSON formatter, it will pick up the trace context from the dummy span that contains the trace context.

Can you elaborate on this?

[agocs]

Can you elaborate on this?

The Python lambda library creates a dummy span that contains the trace context in https://github.com/DataDog/datadog-lambda-python/pull/155/files#diff-eb1f51efb218a7a829e913e4ff6b17ce27be8a777bc20c283e5b843e5db151efL304-L325

If you are logging using the json-log-formatter as recommended in the logs_collection/python docs, that trace context is added to the logs automatically. The logging.Formatter that is created on line 341/344 gets the logs context from that dummy span (or, I suppose, the actual span, but it's the same thing either way).

[nhinsch]

LGTM