pmm-admin summary doesn't include client exporter and some other logs

User story:

• As a PMM user, I need to have logs of pmm-agent and all exporters and other components managed by pmm-agent in Zip file we generate by pmm-admin summary command so that file has much more information and simplifies support cases investigations.  

UI(CLI):

#pmm-admin summary

Will include pmm-agent.log file inside

Acceptance criteria:

• recent logs of all exporters included in "summary zip" together with pmm-agent, vmagent, and other possible components managed by pmm-agent 
• logs from all binaries managed by pmm-agent  included into one file in zip archive returned by 'pmm-admin summary'  

Original report:

The pmm-admin summary outputs for the client don't contain output logs for pmm-agent itself:

shell> ls -1
list.txt
pmm-admin-version.txt
pmm-agent-config.yaml
pmm-agent-version.txt
status.json 

An output like the following should be included:

journalctl -u pmm-agent

Tested with:

shell> pmm-admin --version
ProjectName: pmm-admin
Version: 2.4.0
PMMVersion: 2.4.0
Timestamp: 2020-03-18 12:47:10 (UTC)
FullCommit: bebe0b245b660eb6328f5a8ddb5b8805ca234dac 

Suggested implementation:
Ring buffer implementation example in go: https://github.com/smallnest/ringbuffer

Logrus hook: https://github.com/sirupsen/logrus

1. Add endpoint "/logs.zip" in pmm-agent to get logs.
   1. Response should be a zip file
2. pmm-agent endpoint should also add logs from all running binaries.
   1. Keep them in one file.
3. Store all logrus logs related to pmm-agent in ring buffer.
4. Add request to get logs from pmm-agent endpoint in summary command in pmm-admin.
5. Move all log files from logs.zip into logs directory in pmm-admin-summary.zip.
   1. Use the same approach like we use here https://github.com/percona/pmm-managed/blob/0e1553888d785269ea7eca0c3daa7a6f78eef9a2/services/supervisord/logs.go#L314

How to test:

• Run pmm-agent
• Add mysql and mongodb service
• Run pmm-admin summary command
• Check that ~/pmm-agent/ folder in logs ZIP file contains logs of pmm-agent
• Check that ~/pmm-agent/ folder in logs ZIP file contains following logs:

• • mysqld_exporter logs
  • mongodb_exporter logs
  • qan agent logs
  • pmm-agent log
    **