Fixes #3134 - Improve the performance of the /stats/requests endpoint

[orf]

With the following pathological locustfile, the following change hugely reduces the time it takes to call the /stats/requests endpoint.

import uuid
from locust import FastHttpUser, between, task 

class User(FastHttpUser):
    wait_time = between(1, 5)

    @task
    def call(self):
        self.client.get(
            f"/some-endpoint?a={uuid.uuid4()}",
            json=None,
        )

With the existing code, the endpoint almost immediately takes >100ms to execute:

Whereas with this branch, it stays around 50ms consistently:

Fixes #3134

[cyberw]

Love it! But leave some version of the comment in place, to explain why we're truncating.

[orf]

Thanks @cyberw! I've updated the branch to retain the comment