Updates to the position parameter.

[quiiver]

Nan and I did some validating of the recent addition of the position param during out 1-1 and we found a couple things i missed in the code review. sorry!

• position is part of the message payload, not a top level key in the pubsub message.
• the extraction key is position
• position should only be set for topsite impressions.

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.04% :warning:

Comparison is base (50a4b96) 86.29% compared to head (5fe97f9) 86.25%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2458 +/- ## ============================================ - Coverage 86.29% 86.25% -0.04% - Complexity 895 898 +3 ============================================ Files 123 123 Lines 5166 5167 +1 Branches 517 518 +1 ============================================ - Hits 4458 4457 -1 Misses 547 547 - Partials 161 163 +2 ``` | [Flag](https://app.codecov.io/gh/mozilla/gcp-ingestion/pull/2458/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mozilla) | Coverage Δ | | |---|---|---| | [ingestion_beam](https://app.codecov.io/gh/mozilla/gcp-ingestion/pull/2458/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mozilla) | `82.83% <100.00%> (-0.05%)` | :arrow_down: | | [ingestion_core](https://app.codecov.io/gh/mozilla/gcp-ingestion/pull/2458/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mozilla) | `75.07% <ø> (ø)` | | | [ingestion_sink](https://app.codecov.io/gh/mozilla/gcp-ingestion/pull/2458/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mozilla) | `88.53% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mozilla#carryforward-flags-in-the-pull-request-comment) to find out more. | [Files Changed](https://app.codecov.io/gh/mozilla/gcp-ingestion/pull/2458?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mozilla) | Coverage Δ | | |---|---|---| | [...elemetry/contextualservices/ParseReportingUrl.java](https://app.codecov.io/gh/mozilla/gcp-ingestion/pull/2458?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mozilla#diff-aW5nZXN0aW9uLWJlYW0vc3JjL21haW4vamF2YS9jb20vbW96aWxsYS90ZWxlbWV0cnkvY29udGV4dHVhbHNlcnZpY2VzL1BhcnNlUmVwb3J0aW5nVXJsLmphdmE=) | `78.19% <100.00%> (+0.10%)` | :arrow_up: | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/mozilla/gcp-ingestion/pull/2458/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mozilla)

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