[DOC] Update math in PQ size example

[a03nikki]

Corrected PQ multiplication factor values for JSON documents and the example.

PREVIEW: https://logstash_bk_16411.docs-preview.app.elstc.co/guide/en/logstash/master/persistent-queues.html

Release notes

[rn:skip]

What does this PR do?

Corrects the math in a number of places in the documentation at https://www.elastic.co/guide/en/logstash/8.15/persistent-queues.html.

Why is it important/What is the impact to the user?

The numbers listed on the page did not align with the values when plugged into a calculator. This could lead to confusion when trying to replicate the math.

 | Plaintext size (bytes) | Serialized {ls} event size (bytes) | Overhead (bytes) | Overhead (%) | Multiplication Factor
-| 11 | 213 | 202 | 1836% | 19.4
+| 11 | 213 | 202 | 1836% | 18.36

202/11 = 18.3636363636

| JSON document size (bytes) | Serialized {ls} event size (bytes) | Overhead (bytes) | Overhead (%) | Multiplication Factor
-| 6751 | 7388 | 637 | 9% | 1.9
-| 58901 | 59693 | 792 | 1% | 1.1
+| 6751 | 7388 | 637 | 9% | 1.09
+| 58901 | 59693 | 792 | 1% | 1.01

637/6751 = 0.09435639165
792/58901 = 0.01344629123

*Example*

-Let's consider a {ls} instance that receives 1000 EPS and each event is 1KB,
-or 3.5GB every hour. In order to tolerate a downstream component being unavailable
+Let's consider a {ls} instance that receives 1000 EPS and each event is 1KB (1024 bytes),
+which is 1000 KB per second or 3.5GB (1*1000*3600 KB) every hour.
+In order to tolerate a downstream component being unavailable
 for 12h without {ls} exerting back-pressure upstream, the persistent queue's
-`max_bytes` would have to be set to 3.6*12*1.10 = 47.25GB, or about 50GB.
+`max_bytes` would have to be set to 3.5*12*1.10 = 46.2GB, or about 50GB.

1000 events per second * 1KB = 1000 KB per second
1000 KB *60 seconds per minute * 60 minutes per hour = 3,600,000 KB pr hour
3,600,000 / 1024 / 1024 = 3.4332275391 GB per hour

Round up as we are dealing with disk space to 3.5GB per hour.

3.5GB per hour * 12 hours * 1.10 default multiplication factor = 46.2 GB

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files (and/or docker env variables)
• I have added tests that prove my fix is effective or that my feature works

Author's Checklist

• Double check math and grammar

How to test this PR locally

Redo the math yourself.

Related issues

N/A

Use cases

User trying to determine how much disk space is needed for PQ and doing the calculations based on the documentation.

Screenshots

Logs

N/A

[karenzone]

Builds cleanly and LGTM.

[karenzone]

@robbavey, keep me honest here please?

[robbavey]

@a03nikki @karenzone Do we need to be concerned about KiB vs KB and GiB vs GB here in the calculations and numbers?

I prefer to use the right terms where appropriate, but Logstash itself only takes in gb, etc, and translates them to GiB so presenting this is interesting...