Expose Db and Http on Span.Context via the Public Agent API

[gregkalapos]

Finishing #124.

We already expose Transaction.Context (and stuff on Context), that was done in #134.

In this PR we expose Span.Context (and stuff on `Context).

The approach is the same: as agreed in #124 we simply expose the API defined by the APM Server. No intermediate layer, just a 1-1 mapping of the server json schema to .NET.

[codecov-io]

Codecov Report

Merging #149 into master will increase coverage by 0.74%.
The diff coverage is 97.36%.

@@            Coverage Diff             @@
##           master     #149      +/-   ##
==========================================
+ Coverage   83.99%   84.74%   +0.74%     
==========================================
  Files          41       45       +4     
  Lines        1281     1324      +43     
  Branches      210      216       +6     
==========================================
+ Hits         1076     1122      +46     
+ Misses        124      121       -3     
  Partials       81       81

Impacted Files	Coverage Δ
...astic.Apm/Config/EnvironmentConfigurationReader.cs	100% <100%> (ø)
src/Elastic.Apm/Config/ConfigConsts.cs	100% <100%> (ø)	⬆️
src/Elastic.Apm/Api/Request.cs	100% <100%> (ø)	⬆️
...c/Elastic.Apm.AspNetCore/ApmMiddlewareExtension.cs	45.16% <100%> (ø)	⬆️
src/Elastic.Apm.AspNetCore/ApmMiddleware.cs	87.14% <100%> (+3.8%)	⬆️
src/Elastic.Apm/Api/Response.cs	100% <100%> (ø)	⬆️
...Apm.AspNetCore/Config/MicrosoftExtensionsConfig.cs	77.5% <100%> (+1.18%)	⬆️
.../Elastic.Apm/Config/AbstractConfigurationReader.cs	65.48% <80%> (+1.59%)	⬆️
src/Elastic.Apm/Api/Database.cs	100% <0%> (ø)
... and 5 more

---

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[SergeyKleyman]

Redundant new line :) ?

[gregkalapos]

👍 fixed

[gregkalapos]

Retriggering build.

Sometimes on Windows the installation of the .net tools seems to fail and our builds fail with

[2019-03-20T14:52:21.830Z] C:\Users\jenkins\workspace\tnet_apm-agent-dotnet-mbp_PR-149\src\github.com\elastic\apm-agent-dotnet>dotnet build 
[2019-03-20T14:52:21.830Z] 'dotnet' is not recognized as an internal or external command,
[2019-03-20T14:52:21.830Z] operable program or batch file.

cc @kuisathaverat

[kuisathaverat]

It fails to download the dotnet zip file, I'll change the wait it is installed because I found another better, I'd make it when I implemented the upload to NuGet.

[2019-03-20T14:52:19.521Z] powershell.exe : Invoke-WebRequest : Unable to read data from the transport connection: An existing connection was forcibly closed by 
[2019-03-20T14:52:19.521Z] At C:\Users\jenkins\workspace\tnet_apm-agent-dotnet-mbp_PR-149\dotnet@tmp\durable-2b201a5c\powershellWrapper.ps1:3 char:1
[2019-03-20T14:52:19.521Z] + & powershell -NoProfile -NonInteractive -ExecutionPolicy Bypass -Comm ...
[2019-03-20T14:52:19.521Z] + ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
[2019-03-20T14:52:19.521Z]     + CategoryInfo          : NotSpecified: (Invoke-WebReque...ibly closed by :String) [], RemoteException
[2019-03-20T14:52:19.521Z]     + FullyQualifiedErrorId : NativeCommandError
[2019-03-20T14:52:19.521Z]  
[2019-03-20T14:52:19.521Z] the remote host.
[2019-03-20T14:52:19.521Z] At C:\Users\jenkins\workspace\tnet_apm-agent-dotnet-mbp_PR-149\dotnet@tmp\durable-2b201a5c\powershellScript.ps1:2 
[2019-03-20T14:52:19.521Z] char:25
[2019-03-20T14:52:19.521Z] + ...             Invoke-WebRequest https://download.visualstudio.microsoft ...
[2019-03-20T14:52:19.521Z] +                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
[2019-03-20T14:52:19.521Z]     + CategoryInfo          : NotSpecified: (:) [Invoke-WebRequest], IOException
[2019-03-20T14:52:19.521Z]     + FullyQualifiedErrorId : System.IO.IOException,Microsoft.PowerShell.Commands.InvokeWebRequestCommand