Add logs support on Kafka receiver

[sincejune]

Description:
Add logs support on Kafka exporter

Link to tracking Issue:
Related to #1331
Related to open-telemetry/opentelemetry-collector-contrib#268
Related to #2943

Testing:

• Added corresponding unit tests according to traces and metrics.
• Tested locally with Kafka 2.11-2.0.0

Documentation:
Update Kafka receiver README and Receiver README.

[codecov]

Codecov Report

Merging #2944 (37e049c) into main (90b2dec) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #2944      +/-   ##
==========================================
+ Coverage   91.58%   91.63%   +0.04%     
==========================================
  Files         312      312              
  Lines       15351    15441      +90     
==========================================
+ Hits        14059    14149      +90     
  Misses        883      883              
  Partials      409      409              

Impacted Files	Coverage Δ
receiver/kafkareceiver/factory.go	100.00% <100.00%> (ø)
receiver/kafkareceiver/kafka_receiver.go	100.00% <100.00%> (ø)
receiver/kafkareceiver/otlp_unmarshaller.go	100.00% <100.00%> (ø)
receiver/kafkareceiver/unmarshaller.go	100.00% <100.00%> (ø)

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[bogdandrutu]

If this is private means you don't expect users to implement? Also probably we should rename the previous one to TracesUnmarshaller.

[sincejune]

Thanks, @bogdandrutu. Fixed in 0cb875f

And actually, there're many methods to be renamed (adding Traces). I would like to do this in another PR cuz it's already not a small one. Thoughts?

[bogdandrutu]

Names are inconsistent, probably worth updating the previous one to include traces.

Also I would change the API to:

func WithLogsUnmarshallers(encoding string, unmarshaller logsUnmarshaller) FactoryOption {}

Or even consider to add a name func on the interface to simplify usage.

[bogdandrutu]

Actually there is an Encoding so we can do:

func WithLogsUnmarshallers(unmarshallers logsUnmarshaller...) FactoryOption {}

[sincejune]

The method is actually consistent with the traces' method here:

opentelemetry-collector/receiver/kafkareceiver/factory.go

Lines 47 to 54 in 6458914

	// WithAddUnmarshallers adds marshallers.
	func WithAddUnmarshallers(encodingMarshaller map[string]Unmarshaller) FactoryOption {
	return func(factory *kafkaReceiverFactory) {
	for encoding, unmarshaller := range encodingMarshaller {
	factory.unmarshalers[encoding] = unmarshaller
	}
	}
	}

I'd like to keep the parameters consistent with trace's method and there are multiple encodings and return one factory in traces' method. But I'd like to rename the method name to be consistent in a subsequent PR. Thoughts?