fix(profiling): serialisable stack key

[P403n1x87]

Ensure that the stack key used to group events is actually hashable.

Testing strategy

This is an attempt to mitigate #8218, for which we don't have a reproducer. Should the issue occur again, we should be able to act on any future reports with the extra context that this proposed change provides.

Checklist

• Change(s) are motivated and described in the PR description
• Testing strategy is described if automated tests are not included in the PR
• Risks are described (performance impact, potential for breakage, maintainability)
• Change is maintainable (easy to change, telemetry, documentation)
• Library release note guidelines are followed or label changelog/no-changelog is set
• Documentation is included (in-code, generated user docs, public corp docs)
• Backport labels are set (if applicable)
• If this PR changes the public interface, I've notified @DataDog/apm-tees.
• If change touches code that signs or publishes builds or packages, or handles credentials of any kind, I've requested a review from @DataDog/security-design-and-guidance.

Reviewer Checklist

• Title is accurate
• All changes are related to the pull request's stated goal
• Description motivates each change
• Avoids breaking API changes
• Testing strategy adequately addresses listed risks
• Change is maintainable (easy to change, telemetry, documentation)
• Release note makes sense to a user of the library
• Author has acknowledged and discussed the performance implications of this PR as reported in the benchmarks PR comment
• Backport labels are set in a manner that is consistent with the release branch maintenance policy

[sanchda]

Saw the issue--makes sense. Thanks!

[github-actions]

The backport to 2.5 failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-2.5 2.5
# Navigate to the new working tree
cd .worktrees/backport-2.5
# Create a new branch
git switch --create backport-8435-to-2.5
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 7ff0057d219ab3e5cb9ca8a4283b5199d73bb0aa
# Push it to GitHub
git push --set-upstream origin backport-8435-to-2.5
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-2.5

Then, create a pull request where the base branch is 2.5 and the compare/head branch is backport-8435-to-2.5.

[github-actions]

The backport to 2.6 failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-2.6 2.6
# Navigate to the new working tree
cd .worktrees/backport-2.6
# Create a new branch
git switch --create backport-8435-to-2.6
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 7ff0057d219ab3e5cb9ca8a4283b5199d73bb0aa
# Push it to GitHub
git push --set-upstream origin backport-8435-to-2.6
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-2.6

Then, create a pull request where the base branch is 2.6 and the compare/head branch is backport-8435-to-2.6.

[github-actions]

The backport to 2.7 failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-2.7 2.7
# Navigate to the new working tree
cd .worktrees/backport-2.7
# Create a new branch
git switch --create backport-8435-to-2.7
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 7ff0057d219ab3e5cb9ca8a4283b5199d73bb0aa
# Push it to GitHub
git push --set-upstream origin backport-8435-to-2.7
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-2.7

Then, create a pull request where the base branch is 2.7 and the compare/head branch is backport-8435-to-2.7.