Explain/remove 512 limit of tracestate in spec about trace-context HOT 8 CLOSED

We explained in PR why we need limit. We still haven't explained why the limit is 512

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So far asking different providers - everybody wants smaller limit, but have not upper-bound physical limitation that will prevent saving and propagation of 512 characters.

So we need to put rationaly of why we need 512. If you implemented tracestate - can you post example/size you need?

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For example, B3 encoding will require 72 characters tracestate: b3=80f198ee56343ba864fe8b2a57d3eff7-05e3ac9a4f6e3b90-1-e457b5a2e4d86bd1 (see: openzipkin/b3-propagation#21).

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I feel that any limit number is going to be largely arbitrary. We can instead clearly outline procedure for trimming the state and not specify a hard limit, or use an external one like limit for cookies.

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One thing I think we can avoid by defining some hard limit is abuse of the header to propagate user-defined baggage. If you know that it will be cut anyway - you'd look for solution like Correlation-Context from the beginning instead of frustration when context will not be propagated,

I like that idea though. Definitely good idea to detail the trimming procedure

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I just think the exact limit can be very application specific, e.g. an application that exchanges many small messages will be more sensitive to the header size than one that carries 20Kb JSON requests. So the exact limit might be configurable per application, but the spec's hard limit should be more generous, like the cookie size limit of 4093.

Let the market forces pressure vendors to keep the size to a minimum.

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Here is how the Dynatrace tracestate will likely (typically) look like:

fw5-29a-3039@dt=1;FEC354CD6;1;2;727c (36 bytes)

However, the elements are of variable length, so they could be longer. The theoretical maximum would be like this:

fw5-ffffffff-ffffffff@dt=ffffffff;ffffffffffffffff;ffffffff;ffffffff;ffffffff (77 bytes)

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Closing in favor of #239

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