Ensure dummy metadata segment is json serializable.

[purple4reina]

Fixes error TypeError: Object of type Context is not JSON serializable #402

What does this PR do?

Fixes #402

When creating the xray metadata subsegment, we must pass a json serializable object. Unfortunately, a ddtrace.context.Context object is not json serializable.

Because the create_dd_dummy_metadata_subsegment method is used in other locations as well, we must convert the ddtrace.context.Context object before calling this method.

Motivation

Testing Guidelines

The test that I added passes with v82 of the layer but fails with v83 and v84. The fact that it again passes indicates to me that this fix will work.

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)

[joeyzhao2018]

LGTM