feat: Allow custom prometheus info metric

[rafvasq]

Motivation

Allow the generation of an "info" metric by way of environment variables captured as Gauge metric set up during container startup

Modifications

• Added environment variable MMESH_CUSTOM_ENV_VAR in ModelMeshEnvVars
• Added map infoMetricParams and logic to parse and pass the info to prometheus in ModelMesh
• Added a Gauge using the parsed label names and values in Metrics
• Added sample variables to pom.xml for testing and related test in ModelMeshMetricsTest

Result

• Support to pass information by via environment variables parsed as <metricname>[;label1=envVarWithValueforLabel1,label2=envVarWithValueforLabel2,...,labelN=envVarWithValueforLabelN,] if provided.

[njhill]

@rafvasq it doesn't look like you addressed my earlier comment about resolving env vars.

[njhill]

I don't think it's a great idea to rely on the fact that the passed in hashmap is ordered. For a regular hashmap there's no guarantee these names/values will end up in the same order. Better to create two empty arrays of the correct size prior to the loop above and just populate the names/values in the loop.

[rafvasq]

I was using a linkedhashmap here to preserve the order, I made it a bit more obvious now. Interested to know what you think or if the array creation method would still be better.

[njhill]

Yep I did see the LinkedHashMap but I still think it's cleaner not require it as the type. You could do:

String[] labelNames = infoMetricParams.keySet().toArray(String[]::new);
String[] labelValues = Stream.of(labelNames).map(infoMetricParams::get).toArray(String[]::new);

[rafvasq]

@njhill thanks for the comments and patience, it's ready for another pass. I believe I'm now resolving the env vars as expected.

[njhill]

Thanks @rafvasq I think it's almost there

[njhill]

Yep I did see the LinkedHashMap but I still think it's cleaner not require it as the type. You could do:

String[] labelNames = infoMetricParams.keySet().toArray(String[]::new);
String[] labelValues = Stream.of(labelNames).map(infoMetricParams::get).toArray(String[]::new);

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[njhill]

/lgtm