Make the OpenTelemetry InputFormat More Flexible to Metric, Value and Attribute Types

[marcusgreer]

Fixes #63

Description

• Quietly ignore histogram and summary metrics in the OpenTelemetry InputFormat. Previously an exception would be thrown if a histogram or summary metric were ingested.
• Do not throw an error if DataPoint values are VALUE_NOT_SET, instead use a double of value 0.
• Do not throw an error if Metric Attributes are not strings, instead parse the object based off of the valueCase. If VALUE_NOT_SET, then set to an empty string
• Create a DruidModule because the OpenTelemetryInputFormat could not be deployed as a standalone InputFormat since Druid could not discover the OpenTelemetryExtensionModule.

[xvrl]

instead of creating a new arraylist, we should return an empty list, which avoids unnecessary memory allocation

Suggested change

	inputRows = new ArrayList<>();
	inputRows = Collections.emptyList();

[freyayunfu]

lgtm thanks for fixing this!

[xvrl]

how does this get treated?

[marcusgreer]

Does druid have a set of expected value types for columns? If it does, it would probably make sense for us to do some conversion from the disallowed types into strings or some other type.

[freyayunfu]

just fyi, for converter, we only support int, bool , double and string here: https://github.com/confluentinc/opencensus-protobuf-converter/blob/d96358a0d61f6f3524c3acc9d7db264a7a1a34c8/src/main/java/io/confluent/connect/opentelemetry/OpenTelemetryConverter.java#L91-L93 Not sure whether we want to keep it consistent here. (sorry for merging the PR without noticing the difference here.)

[marcusgreer]

It looks like the OpenCensusProtobufProtobufReader changes all values into strings (LabelValue::getValue here returns a String). So I'll do the same thing here.

[freyayunfu]

the link i provided was for attributes, not metrics value. This should be the correct one: https://github.com/confluentinc/opencensus-protobuf-converter/blob/d96358a0d61f6f3524c3acc9d7db264a7a1a34c8/src/main/java/io/confluent/connect/opentelemetry/OpenTelemetryMetricsConverter.java#L121-L166

[xvrl]

how does empty string get treated if someone uses a numeric column?

[freyayunfu]

It will return 0 as a default value of numeric column. In the code, here is what happens in detail: it would throw below exception first from here

 throw new NumberFormatException("empty String");

and then get caught here, and return default value.

Probably it's still better to return null here since druid would replace with default value according to different column specs directly here instead of going throw the whole parsing process, even though the eventual result is the same.

[xvrl]

should this be OpenTelemetry?