Migrate /v1/bulk/observations/series/linked

[n-h-diaz]

This PR migrates usage of /v1/bulk/observations/series/linked to /v2/observation, including:

• Migration testing: Removed reference in tests
• Load testing: Updated test cases to use comparable V2Observation requests. Also updated the test to accept an API key. The mixer-loadtester pod is running on dev. Tested using this instance.
• Memory profiling: This used the mixer RPC BulkObservationsSeriesLinked. Updated http_memprof test cases to use comparable V2Observation requests and RPC. Tested locally following https://github.com/datacommonsorg/mixer/blob/master/docs/developer_guide.md#profile-api-requests-against-a-running-mixer-instance

Note based on gemini comments (more details in comments below): I had to update the test cases a bit, including

• Changing request params to fit the V2 series limit
• Not including preferred facets, since V2 doesn't have this concept

[n-h-diaz]

/gemini review

[gemini-code-assist]

Code Review

This PR migrates usage of /v1/bulk/observations/series/linked to /v2/observation. The changes look good overall, covering migration testing, load testing, and memory profiling. I've found a few issues related to the comparability of the new test cases with the old ones they are replacing. Specifically, some entity expressions in the memory profiling and load testing scripts do not match the original V1 requests. Additionally, the memory profiling test has reduced coverage as it no longer tests with and without facets. I've left detailed comments with suggestions for these points.