[DEPR-2U]: Remove md4 caching and related ENABLE_BLAKE2B_HASHING feature flag

[robrap]

We need to enable this feature flag in advance of its removal, detailed in the following DEPR:

• [DEPR]: Remove md4 caching and related ENABLE_BLAKE2B_HASHING feature flag openedx/edx-platform#36024

Tim did mention possibly adding telemetry to better understand the impact before enabling in this comment on the PR where the toggle had been added:

• fix: replaced md4 with blake2b openedx/edx-platform#34442 (comment)

Looking to complete this by April 2025.

Acceptance Criteria:

• Enable the flag in all environments
  • https://github.com/edx/edx-internal/pull/12090 (stage)
  • https://github.com/edx/edge-internal/pull/819 (edge)
  • https://github.com/edx/edx-internal/pull/12092 (prod)
• Communicate on DEPR when we have completed this.
• Ensure our toggles are removed once the DEPR is complete

Implementation Tasks:

• Consider adding telemetry to better understand the impact before enabling.
  • Add monitoring temp: Add temporary monitoring for large memcache keys openedx/edx-platform#36034
  • Remove monitoring once we have the data we need

[robrap]

First pass at instrumentation:

• increment('safe_key_called') # count of all calls
• increment('safe_key_with_hash') # count of all calls taking the hashing branch

I can't remember if we'll actually be able to sum this information in DD? You might need to create an integer-based facet based on the span tag.

Based on these findings, we may want logging with the actual keys, or we may feel it is safe enough if the with-hash percentage is low enough.

[timmc-edx]

Monitoring notebook: https://app.datadoghq.com/notebook/10845716/migration-of-md4-to-blake2b-in-memcache-key-hashing?notebookType=legacy

[pdpinch]

FYI, the PR to remove the flag has been opened at openedx/edx-platform#36054

We won't merge it until y'all have resolved & closed this issue.

[timmc-edx]

Thanks! We're actually ready for this to proceed -- the remainder of this ticket is just communication and post-removal cleanup.

[pdpinch]

Ok, the PR to remove the flag has been merged. Let me know if there's anything else we can do to help.