[exporter/splunkhec] enabled http2 healthcheck

[dloucasfx]

Description:
Same description as in open-telemetry/opentelemetry-collector#9022

This PR enables the HTTP2 health check to workaround the issue described here open-telemetry/opentelemetry-collector#9022

As to why I chose 10 seconds for HTTP2ReadIdleTimeout and 10 seconds for HTTP2PingTimeout
Those values have been tested in production and they will result, in an active env (with default http timeout of 10 seconds and default retry settings), of a single export failure or (2 max) before the health check detects the corrupted tcp connection and closes it.
The only drawback is if the connection was not used for over 10 seconds, we might end up sending unnecessary ping frames, which should not be an issue and if it became an issue, then we can tune those settings.

The SFX exporter has multiples http clients:

• Metric client, Trace client and Event client . Those client will have the http2 health check enabled by default as they share the same default config
• Correlation client and Dimension client will NOT have the http2 health check enabled. We can revisit this if needed.

Link to tracking Issue:

Testing:

• Run OTEL with one of the exporters that uses HTTP/2 client, example signalfx exporter
• For simplicity use a single pipeline/exporter
• In a different shell, run this to watch the tcp state of the established connection

 while (true); do echo date; sudo netstat -anp | grep -E '<endpoin_ip_address(es)>' | sort -k 5; sleep 2; done

• From the netstat, take a note of the source port and the source IP address
• replace <> from previous step
  sudo iptables -A OUTPUT -s <source_IP> -p tcp --sport <source_Port> -j DROP
• Note how the OTEL exporter export starts timing out

Expected Result:

• A new connection should be established, similarly to http/1 and exports should succeed

Actual Result:

• The exports keep failing for ~ 15 minutes or for whatever the OS tcp_retries2 is configured to
• After 15 minutes, a new tcp connection is created and exports start working

Documentation:
Readme is updated

Disclaimer:
Not all HTTP/2 servers support H2 Ping, however, this should not be a concern as our ingest servers do support H2 ping.
But if you are routing you can check if H2 ping is supported using this script golang/go#60818 (comment)