RFR: 7506: Incorrect numeric formatting of PID by JMC

[Suchita Chaturvedi]

This PR is to fix the formatting issue of the JVM PID on JVM information screen. JVM PID attribute is received as NUMBER from JFR and hence the JMC application is treating is as number. When the number is smaller , the PID is displayed using commas and when the number is larger, its converted to exponential format. This is a long pending bug in product.

The PID (Process Id) attribute is present in JMC as part of System Process event also where the data type is correct i.e. String. So in order to maintain consistency, we have made the content type of PID attribute of JVM Information event also as String. We are converting the number value of PID to string and then storing it. 

We have also introduced a new aggregator PID which will contain the value of Identifier in plain text format instead of number format. This new aggregator is displayed on JVM Information page.

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Commit messages:
 - 7506: Incorrect numeric formatting of PID by JMC

Changes: https://git.openjdk.org/jmc/pull/565/files
  Webrev: https://webrevs.openjdk.org/?repo=jmc&pr=565&range=00
  Issue: https://bugs.openjdk.org/browse/JMC-7506
  Stats: 46 lines in 3 files changed: 40 ins; 1 del; 5 mod
  Patch: https://git.openjdk.org/jmc/pull/565.diff
  Fetch: git fetch https://git.openjdk.org/jmc.git pull/565/head:pull/565

PR: https://git.openjdk.org/jmc/pull/565