Logically separate out parser to its own package for import of metric, event, and service check message parsing

[schahal]

What does this PR do?

Moves the parser logic into its own sub-dogstatsd package and makes the following functions exportable:

• NextMessage(packet *[]byte) (message []byte)
• ParseServiceCheckMessage(message []byte, defaultHostname string) (*metrics.ServiceCheck, error)
• ParseEventMessage(message []byte, defaultHostname string) (*metrics.Event, error)
• ParseMetricMessage(message []byte, namespace string, defaultHostname string) (*metrics.MetricSample, error)

...which are called by the dogstatsd server.

Motivation

Mainly, so we can call the parsing functions without having to pull in the server/listener weight. It can also be advantageous in the future should there be different implementations of the dogstatsd server: it'd be able to call the same underlying parsing funcs without duplicating code.

Also, the separation also seems cleaner/logical for hopefully improved maintainability.

Additional Notes

Please note, I did not update releasenotes (per contribution guidelines) as there is no change to the end-server, just code layout. In case I should, please let me know and I'll update the PR.

Go Test Results

server_test.go

[~/go/src/github.com/DataDog/datadog-agent/pkg/dogstatsd] $ go test
Error: Dogstatsd: error parsing metrics: invalid metric value for "daemon1:666:777|g"
Error: Dogstatsd: error parsing service check: invalid field number for "_sc|agen.down"
Error: Dogstatsd: error parsing event: Invalid event message format: empty 'title' or 'text' field
PASS
ok  	github.com/DataDog/datadog-agent/pkg/dogstatsd	0.365s

parser_test.go

[~/go/src/github.com/DataDog/datadog-agent/pkg/dogstatsd] $ cd parser/ && go test
PASS
ok  	github.com/DataDog/datadog-agent/pkg/dogstatsd/parser	0.008s

[olivielpeau]

Hi @schahal, thanks for opening this PR, and sorry we failed to respond earlier.

I see the related issue on telegraf has been closed, so I don't know if your PR is still something you'd want to see merged. In addition, a few things were refactored/moved in the dogstatsd package since you opened your PR so I don't think it'll be easy to rebase and update. So I'll go ahead and close your PR, but if this something you're still interested in feel free to follow up here, thanks!