chore: add contextual attributes to faro measurement

[codecapitano]

PR Description

In the Faro Web-SDK we added an option to provide additional contextual attributes to the measurements payload.
To enable this we need to update the agent as well.

Which issue(s) this PR fixes

Notes to the Reviewer

Just for reference here are the PRs which make the needed changes to the Faro Receiver and the Faro Web-SDK.

• PR Receiver
• PR Docs

PR Checklist

• CHANGELOG.md updated
• Tests updated

[rlankfo]

LGTM for the MeasurementContext. I had one question about adding Type but I'm not sure if it's necessary in this PR.

[rlankfo]

@codecapitano looks like there are some additional test cases we should update:

--- FAIL: TestExportLogs (0.01s)
    logs_exporter_test.go:103: 
        	Error Trace:	/Users/runner/work/agent/agent/pkg/integrations/v2/app_agent_receiver/logs_exporter_test.go:103
        	Error:      	Not equal: 
        	            	expected: "timestamp=\"2021-09-30 10:46:17.68 +0000 UTC\" kind=measurement ttfb=14.000000 ttfcp=22.120000 ttfp=20.120000 traceID=abcd spanID=def sdk_name=grafana-frontend-agent sdk_version=1.0.0 app_name=testapp app_release=0.8.2 app_version=abcdefg app_environment=production user_email=geralt@kaermorhen.org user_id=123 user_username=domasx2 user_attr_foo=bar session_id=abcd session_attr_time_elapsed=100s page_url=https://example.com/page browser_name=chrome browser_version=88.12.1 browser_os=linux browser_mobile=false view_name=foobar"
        	            	actual  : "timestamp=\"2021-09-30 10:46:17.68 +0000 UTC\" kind=measurement ttfb=14.000000 ttfcp=22.120000 ttfp=20.120000 traceID=abcd spanID=def context_hello=world sdk_name=grafana-frontend-agent sdk_version=1.0.0 app_name=testapp app_release=0.8.2 app_version=abcdefg app_environment=production user_email=geralt@kaermorhen.org user_id=123 user_username=domasx2 user_attr_foo=bar session_id=abcd session_attr_time_elapsed=100s page_url=https://example.com/page browser_name=chrome browser_version=88.12.1 browser_os=linux browser_mobile=false view_name=foobar"
        	            	
        	            	Diff:
        	            	--- Expected
        	            	+++ Actual
        	            	@@ -1 +1 @@
        	            	-timestamp="2021-09-30 10:46:17.68 +0000 UTC" kind=measurement ttfb=14.000000 ttfcp=22.120000 ttfp=20.120000 traceID=abcd spanID=def sdk_name=grafana-frontend-agent sdk_version=1.0.0 app_name=testapp app_release=0.8.2 app_version=abcdefg app_environment=production user_email=geralt@kaermorhen.org user_id=123 user_username=domasx2 user_attr_foo=bar session_id=abcd session_attr_time_elapsed=100s page_url=https://example.com/page browser_name=chrome browser_version=88.12.1 browser_os=linux browser_mobile=false view_name=foobar
        	            	+timestamp="2021-09-30 10:46:17.68 +0000 UTC" kind=measurement ttfb=14.000000 ttfcp=22.120000 ttfp=20.120000 traceID=abcd spanID=def context_hello=world sdk_name=grafana-frontend-agent sdk_version=1.0.0 app_name=testapp app_release=0.8.2 app_version=abcdefg app_environment=production user_email=geralt@kaermorhen.org user_id=123 user_username=domasx2 user_attr_foo=bar session_id=abcd session_attr_time_elapsed=100s page_url=https://example.com/page browser_name=chrome browser_version=88.12.1 browser_os=linux browser_mobile=false view_name=foobar
        	Test:       	TestExportLogs

[rlankfo]

test cases + changelog formatting

[rlankfo]

lgtm!