feat(measurements): Add experimental set_measurement api on transaction

[sl0thentr0py]

I went with set_measurement(name, value, unit) (singular) instead of set_measurements as in JS since I think this is more convenient for the end user. Otherwise they need to build the name -> unit/value dict themselves.

This should be merged once develop is synced and the changes in relay have been made to support units for measurements.

[antonpirker]

I like this better then the plural version. Very clean interface. This way one can set and update values.

[marandaneto]

if I do:

child = transaction.start_child(x)

can I do child.set_measurement(x, y) too?

Or start_child would return a different interface that does not provide set_measurement? since measurements are only available at the transaction level.

[sl0thentr0py]

Currently they are just on transaction and not span, start_child returns a span so you can't add measurements there. From what I understood of the requirements, we only want to expose this api on the transaction but this is a design decision and we can also change it.

Open decision is also if we want to expose this on the hub/top-level api as well where it'll just fetch the current hub/scope/transaction and set it there.

[marandaneto]

I agree that we should not leak this API to the Child level as well, but a few SDKs return the very same interface for start_transaction and start_child, so unless we change that, the public API will leak.
cc @bruno-garcia

[github-actions]

This pull request has gone three weeks without activity. In another week, I will close it.

But! If you comment or otherwise update it, I will reset the clock, and if you label it Status: Backlog or Status: In Progress, I will leave it alone ... forever!

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"A weed is but an unloved flower." ― Ella Wheeler Wilcox 🥀

[sl0thentr0py]

tested end-end with relay/ingestion

https://sentry.io/api/0/projects/sentry-sdks/sentry-python/events/547beff9de5445d9953de3b802adf2d0/json/

[sl0thentr0py]

tested graceful error handling (unit name too long)