This commit adds sleep in `close()` for metrics and/or traces to be
flushed before closing observability.

Currently sleep is set to 2 * [Metrics export interval (30 secs)].

Everywhere else is using
androidx.test.ext.junit.runners.AndroidJUnit4, and google internally
only has that variant.

This will be used to generate more useful debugging information in
cases such as headers size exceeding the limit.

Add an example using waitForReady

Part of fixit.  Fixes b/259286751

This PR adds a default custom tag for metrics, irrespective of custom
tags being present in the observability configuration. 

OpenCensus by default adds a custom tag
[opencenus_task](https://docs.google.com/document/d/1sWC-XD277cM0PXxAhzJKY2X0Uj2W7bVoSv-jvnA0N8Q/edit?resourcekey=0-l-wqh1fctxZXHCUrvZv2BQ#heading=h.xy85j580eik0)
for metrics which gets overriden if custom tags are set.

The unique custom tag is required to ensure the uniqueness of the
Timeseries. The format of the default custom tag is:
`java-{PID}@{HOSTNAME}`, if `{PID}` is not available a random number
will be used.

… in close() (grpc#9977)

After grpc#9972, the `sleep()` is done inside Observability `close()`, we can remove this `sleep()` in the Observability Interop test binary.

* add examples for gcp-observability

It uses the echo service for both unary and bidi RPCs, to show the
various cancellation circumstances and APIs.

* Added s390x platform support
* Adapt to existing platform naming scheme
* Updated s390_64 library whitelist
* Use g++ compiler version 8.x for s390x
* Introduced dedicated Docker container for building s390x artifacts Minor fix

---------

Signed-off-by: Dirk Haubenreisser <haubenr@de.ibm.com>
Co-authored-by: Eric Anderson <ejona@google.com>

Add round robin to the example.

fixed