[GOBBLIN-2164] Add default value for OpenTelemetry attribute

[khandelwal-prateek]

Dear Gobblin maintainers,

Please accept this PR. I understand that it will not be reviewed until I have checked off all the steps below!

JIRA

• My PR addresses the following Gobblin JIRA issues and references them in the PR title. For example, "[GOBBLIN-XXX] My Gobblin PR"
  • https://issues.apache.org/jira/browse/GOBBLIN-2164

Description

• Here are some details about my PR, including screenshots (if applicable):
  • As per the Metric Data Model (MDM) documentation, it is essential to define pre-aggregation rules for metrics to allow for effective slicing and dicing based on dimensions. When emitting metrics, if any dimension within a pre-aggregation does not have a value (i.e., it is an empty string ""), the metric will be dropped as it is treated as having no value, which prevents accurate metric reporting and analysis.
  • Added default value to ensure that all dimensions for pre-aggregated metrics are populated with either their actual values or a default placeholder value - if they are empty.

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:

Commits

• My commits all reference JIRA issues in their subject lines, and I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"