Use sync.Pool to reduce lengthReader allocations

[francislavoie]

In #5461 we introduced lengthReader to count the amount of bytes read from the request body.

We're allocating a new lengthReader on each request though (but only requests with a body, so not GETs). This is inefficient, it could be improved by using sync.Pool to reuse allocated lengthReader structs.

[perhapsmaple]

Hi @francislavoie, I would like to work on this issue. Are there any benchmarks that I can use to verify the performance impact of the change? Thanks!

[mholt]

You can write a simple benchmark test to measure allocations and such. 👍

[shashank-priyadarshi]

Hi, is this issue being worked upon? I can take it up otherwise.

[francislavoie]

PRs are always welcome 👍

[shashank-priyadarshi]

@perhapsmaple could you please let me know if you are still working on this? If not, can I please take this up!