No options to scale gRPC server

[yatindra]

Describe the bug
io.vertx.grpc.VertxServer do not auto-scale across Vert.x event-loop. In Vert.x the scaling is determined by the number of deployed instances of a Verticles. I am not entirely familiar with how Quarkus initializes the GrpcServerRecorder, but it seems like only one instance of VertxServer is ever started.

Expected behavior
Singletons implementing a gRPC Service should be allowed to scale across Vert.x event-loop. Additionally, provide a way to configure the desired scaling.

Actual behavior
At most 2 Vert.x event loop threads process request for VertxServer

To Reproduce
Steps to reproduce the behavior:

1. Implement a Quarkus gRPC server with one service annotated with @Singleton and extending MutinyApiConfigServiceGrpc.[GrpcServiceName]ImplBase
2. In the service, write a log line to print the Thread Name
3. Build and run the server
4. Make RPC calls. Used ghz.sh to simulate load.
5. Notice in the logs that only 2 unique thread names are printed. Threads will have the prefix vert.x-eventloop-thread-.

Configuration
No additional configuration were provided. gRPC extension was using the default values.

Environment (please complete the following information):

• CPU count: 8
• Vert.x Event Loop thread count: 16 (the default 2*CPU)
• Output of uname -a or ver:

Darwin REMMACJ5Y8GTFM 19.5.0 Darwin Kernel Version 19.5.0: Tue May 26 20:41:44 PDT 2020; root:xnu-6153.121.2~2/RELEASE_X86_64 x86_64

• Output of java -version:

openjdk version "11.0.2" 2019-01-15
OpenJDK Runtime Environment 18.9 (build 11.0.2+9)
OpenJDK 64-Bit Server VM 18.9 (build 11.0.2+9, mixed mode)

• Quarkus version or git rev: 1.5.1.Final
• Build tool (ie. output of mvnw --version or gradlew --version):

------------------------------------------------------------
Gradle 6.5
------------------------------------------------------------

Build time:   2020-06-02 20:46:21 UTC
Revision:     a27f41e4ae5e8a41ab9b19f8dd6d86d7b384dad4

Kotlin:       1.3.72
Groovy:       2.5.11
Ant:          Apache Ant(TM) version 1.10.7 compiled on September 1 2019
JVM:          11.0.2 (Oracle Corporation 11.0.2+9)
OS:           Mac OS X 10.15.5 x86_64

Additional context
io.quarkus.grpc.runtime.GrpcServerRecorder has the following comment on line 62

// TODO Support scalability model (using a verticle and instance number)

[quarkusbot]

/cc @michalszynkiewicz, @cescoffier

[cescoffier]

I believe there is a TODO in the code explaining that the server should be started in a verticle and the number of verticles should be configurable.