fix(metrics): preserve default_tags when metric-specific tag is set in Datadog provider

[leandrodamascena]

Issue number: #2990

Summary

Changes

In this PR, we address an issue where default_tags were not preserved when adding metric-specific tags in the Datadog. We've implemented a fix to ensure that default_tags will be merged with metric-specific tags. We also fixed the precedence problem when default tags and metric-specific tags shared the same key, and now the metric-specific tags took precedence.

I also added datadog-lambda dependency as optional. We mistakenly removed this in some reviews of the original PR.

User experience

CODE

from aws_lambda_powertools.metrics.provider.datadog import DatadogMetrics, DatadogProvider
from aws_lambda_powertools.utilities.typing import LambdaContext

provider = DatadogProvider(flush_to_log=True)
metrics = DatadogMetrics(provider=provider)
metrics.set_default_tags(product="ticket")


@metrics.log_metrics  # ensures metrics are flushed upon request completion/failure
def lambda_handler(event: dict, context: LambdaContext):
    metrics.add_metric(name="SuccessfulBooking", value=1, flight="AB123")

Output before this fix:

{
    "m": "SuccessfulBooking",
    "v": 1,
    "e": 1692736997,
    "t": [
        "flight:AB123",
    ]
}

Output after this fix:

{
    "m": "SuccessfulBooking",
    "v": 1,
    "e": 1692736997,
    "t": [
        "product:ticket"
        "flight:AB123",
    ]
}

Checklist

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• Meet tenets criteria
• I have performed a self-review of this change
• Changes have been tested
• Changes are documented
• PR title follows conventional commit semantics

Is this a breaking change?

RFC issue number:

Checklist:

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

Acknowledgment

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[rubenfonseca]

stellar!

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
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[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (2a8be25) 96.56% compared to head (e8b3b8e) 96.56%.
Report is 7 commits behind head on develop.

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Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2997   +/-   ##
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  Coverage    96.56%   96.56%           
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  Files          175      175           
  Lines         7825     7825           
  Branches      1476     1476           
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  Hits          7556     7556           
  Misses         217      217           
  Partials        52       52           

Files Changed	Coverage Δ
...bda_powertools/metrics/provider/datadog/datadog.py	100.00% <100.00%> (ø)

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