feat: add capture_cold_start_metric for log_metrics

[heitorlessa]

Issue #, if available:

Description of changes:

This feature allows capturing cold start as a metric with an optional param in log_metrics. It also adds an additional dimension named function_name .

from aws_lambda_powertools.metrics import Metrics, MetricUnit

metrics = Metrics(service="ExampleService")

@metrics.log_metrics(capture_cold_start_metric=True)
def lambda_handler(evt, ctx):
    ...

• Meet tenets criteria
• Update tests
• Update docs
• PR title follows conventional commit semantics

Breaking change checklist

Ignore if it's not a breaking change

RFC issue #:

• Migration process documented
• Implement warnings (if it can live side by side)

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[codecov-commenter]

Codecov Report

Merging #67 into develop will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##           develop       #67   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           15        15           
  Lines          496       508   +12     
  Branches        45        48    +3     
=========================================
+ Hits           496       508   +12     

Impacted Files	Coverage Δ
aws_lambda_powertools/metrics/metrics.py	100.00% <100.00%> (ø)

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[heitorlessa]

If you have ideas of a better name, please suggest - I've also tried emit_cold_start_metric but capture_cold_start_metric read more explicit to me.