Add support for Custom fields

[skynet2]

Can not reopen original issue #116

Per last conversation on APM discussion elastic/apm#7 it was decided to stay with custom.
It will good to have that in APM agent.

Thanks,
Stas

[SergeyKleyman]

@skynet2 Could you please give an example as to why you would like to use custom part of the context and not labels?

[SergeyKleyman]

@skynet2 I see that you already gave an example at #118 (comment)

the reason why i decided to use custom is that it can save big objects, for example, requests and response.

[SergeyKleyman]

BTW I see that Java agent 1.7.0 added support for custom context. Other agents support custom context as well:

• Java
• Node.js
• Python
• Ruby
• Go (it's not clear if Go agent supports custom context)

@axw Does Go agent support custom context?

[skynet2]

Hi @SergeyKleyman ,
Yep, go agent has support for that elastic/apm-agent-go#594 (comment)

[alexandre-sampaio]

Hi! is anyone working on it?
I need this to save the body from response, maybe there is another way, but i didn't find it.

[skynet2]

@alexandre-sampaio for as a temporary solution use Request.Body. According to APM specification it can be both, string or object., so for me its simple key-value pairs.

[gregkalapos]

Implemented in #585 - will be included in the next minor release.