[Feature request] Raise the number of maximum fields

[LukeMathWalker]

Feature Request

Crates

Motivation

At the moment, 32 is the maximum number of fields that can be attached to a span.
While it might seem a lot, at TrueLayer we bumped into this limit several times: a complex business operation will involve several domain-relevant identifiers as well as multiple "technical" fields (e.g. OpenTelemetry required fields according to semantic conventions).

Proposal

I see two main options:

• Make it configurable via a feature flag, similarly to what diesel does for the maximum number of columns. Those who need the feature pay the cost for it, all other users are not impacted;
• Remove the limit entirely, using a growable data structure (which would perhaps unlock Add support for recording dynamic fields #1343 as well?), which I imagine would end up affecting performance by requiring more allocations?

Alternatives

Keep it as it? 🤷🏻‍♂️