do not disable server statistics for agent instances

[jsteemann]

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The arangodb starter seems to set the --server.statistics false parameter for agents, which disables all statistics. The original motivation for this was to turn off a background thread in the instance that periodically writes statistics into RocksDB. This background writing of the statistics history causes load and more disk space usage, which we want to avoid on agent instances.

However, setting --server.statistics false not only disables the background writing of statistics into RocksDB, but also the statistics API and metrics for process statistics.

As it was not ideal to have a single startup option to control two different things, since 3.4.9 arangod provides a separate option --server.statistics-history. This option controls whether the statistics are written to RocksDB. This is what we want to turn off.

The good thing is that when this option is not set, it automatically turns itself off on agents and DB servers.

The defaults in arangod are:

• --server.statistics: always true, for all instance types
• --server.statistics-history: true for coordinators, single servers and active failover instances, false for agents and DB servers.

These are already the optimal settings, so that the arangodb starter does not need to set any statistics-related startup options for arangod.

This PR modifies the starter to not set any of the above arangod startup options, as the arangod defaults seem good enough since at least 3.4.9.