Custom message processor for logs generator

[preet-dev]

Problem Statement

Currently, the log generator has 2 limitations that we want to address:

• As per OpenTelemetry spec, severity is not always mandatory so we need to adjust for that.
• With our without severity in context, we need to provide one or more ways for users to be able to specify custom logs from common sources.

Proposed Solution

• We need to remove mandatory setting for severity expression (adjusting as per OpenTelemetry spec).
• We should finalize a few common source types for log messages and support them.