Improve resource.Quantity to float conversion for SLOs

[levan-m]

What does this PR do?

CONS-6488

Quantity.AsApproximateFloat64 does some math to convert Quantity to float64 needed for SLO API input so some floats may appear with additional decimals. Instead we convert Quantity to Decimal string and then parse as float. This may still have edge cases were string representation doesn't have exact float64 so conversion isn't 1-to-1.

This impl ignores string to float parsing errors, assuming decimal string obtain from Quantity would be valid.

Motivation

What inspired you to submit this pull request?

Additional Notes

Anything else we should know when reviewing?

Minimum Agent Versions

Are there minimum versions of the Datadog Agent and/or Cluster Agent required?

• Agent: vX.Y.Z
• Cluster Agent: vX.Y.Z

Describe your test plan

1. Enabled SLO CRD using datadogSLOEnabled
2. Apply SLO manifest and confirm in the app number match (to see warningThreshold clone the SLO in UI)

apiVersion: datadoghq.com/v1alpha1
kind: DatadogSLO
metadata:
  name: test-slo
  namespace: system
spec:
  description: Error SLO for test-xyz
  name: Error SLO for test-xyz
  query:
    denominator: sum:trace.pyramid.request.hits{service:test-xyz, env:test}.as_count()
    numerator: sum:trace.pyramid.request.hits{service:test-xyz, env:test}.as_count()
      - sum:trace.pyramid.request.errors{service:test-xyz, env:test}.as_count()
  tags:
  - integration:kubernetes
  - service:test-xyz
  - env:test
  - team:sre
  - generated:kubernetes
  targetThreshold: "0.001"
  warningThreshold: "99.999"
  timeframe: 7d
  type: metric

Tested on user-reported combinations

* "99.95" produces 99.95%
* "99.99" produces "error updating SLO" in k8s
* "99.990" produces "error updating SLO" in k8s
* "99.991" produces 99.991%
* 99950m produces 99.95%
* 99990m produces "error updating SLO" in k8s

Checklist

• PR has at least one valid label: bug, enhancement, refactoring, documentation, tooling, and/or dependencies
• PR has a milestone or the qa/skip-qa label

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 55.21%. Comparing base (1c66029) to head (f7cdd02).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1308   +/-   ##
=======================================
  Coverage   55.20%   55.21%           
=======================================
  Files         247      247           
  Lines       28383    28384    +1     
=======================================
+ Hits        15670    15671    +1     
  Misses      11827    11827           
  Partials      886      886           

Flag	Coverage Δ
unittests	55.21% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
controllers/datadogslo/slo.go	61.38% <100.00%> (+0.38%)	⬆️

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