Middlewares should create subsegments only when in the Lambda context running under a Lambda environment

[chanchiem]

The current behavior of the Middlewares is that it opts to generate subsegments only if the current execution environment is in lambda, otherwise it generates segments.

This is problematic if customers choose to create their own custom context and proceed to use that context in a Lambda environment. If for example the custom context is designed to allow users to call put_segment and end_segment, the middlewares would still call begin_subsegment if the context is run in a Lambda environment.

The expected behavior of the middleware is to only generate subsegments if it's in a Lambda context running in the Lambda environment, and if the middleware is run otherwise, it should only generate segments.

[chanchiem]

Case Study: Using the default Context in a Lambda environment

Prior to PR #127, using the default Context (as opposed to the LambdaContext) in a Lambda environment would generate a valid root segment. This happened because the trace ID was propagated from the lambda container and received from the middleware in the SDK to construct a segment whose trace ID matched the Lambda-generated ID. Although this is not a fully supported work around, the introduction of the PR should not change the behavior of what originally happened. The original service graph that used the default Context in Lambda prior to the PR looked like this:

Although this is valid for functional purposes, it doesn't provide the expected service graph because there is no causal relationship between the middleware and the Lambda function.

And for completeness, this is what the Service graph looks like after the PR using the LambdaContext in a Lambda environment as intended:

---

With the introduction of the PR, middlewares generated subsegments inside Lambda environments and caused SegmentMissingExceptions whenever the default Context was used. This issue points out that middlewares should still exhibit the same behavior and attempt to beginSegments whenever a context other than the LambdaContext is used.