feat: provide memstat methods to assist on testing (pipe-538)

[lvrach]

Description

Motivation

The existing method for verifying stats .Get during tests was limiting and cumbersome.

You could only retrieve a specific combination of name and tags. This way:

• You could not test if other metrics were created during the execution. feat: track event delivery stats rudder-server#3974 (comment)
• It is difficult to find the problem if the particular combination of name+tags is missing.

For these reasons, I wanted a way to retrieve a collection of metrics registered to the store.

Solution

• Introduce .GetAll which returns a Metrics array, containing all possible name+stats combinations captured using the storage.
• Introduce .GetByName which returns the subset that belongs to a measurement name.

Metric type is a bit special, it has

• Value just float used for metrics/gauges
• Values float array for histograms
• Durations for timers

The motivation here was to use the most convenient representation for the stat type used. I tried a few approaches:

• using a method per stat type
• using generic interface to describe each Metric

However, both approaches had extra overhead when writing tests. So, I decided to go with the simpler approach from the perspective of validation.

You can seem them being used here

Linear Ticket

https://linear.app/rudderstack/issue/PIPE-538/improve-memstats-testing

Security

• The code changed/added as part of this pull request won't create any security issues with how the software is being used.

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (52204ef) 79.23% compared to head (8afcedf) 79.41%.

Files	Patch %	Lines
stats/memstats/stats.go	94.87%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #210      +/-   ##
==========================================
+ Coverage   79.23%   79.41%   +0.17%     
==========================================
  Files          70       70              
  Lines        5183     5222      +39     
==========================================
+ Hits         4107     4147      +40     
- Misses        873      874       +1     
+ Partials      203      201       -2     

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[achettyiitr]

Thanks @lvrach for picking this up. It would be helpful in scenarios like this.

[fracasula]

Nice one. Just left an optional comment.